With this higher acidity, gastric acid plays a key protective role against pathogens. It is also key in the digestion of proteins by activating digestive enzymes, which together break down the long chains of amino acids. Gastric acid is regulated in feedback systems to increase production when needed, such as after a meal. Other cells in the stomach produce bicarbonate, a base, to buffer the fluid, ensuring a regulated pH. These cells also produce mucus – a viscous barrier to prevent gastric acid from damaging the stomach. The pancreas further produces large amounts of bicarbonate, secreting this through the pancreatic duct to the duodenum to neutralize gastric acid passing into the digestive tract.
The secretion is a complex and relatively energetically expensive
process. Parietal cells contain an extensive secretory network (called canaliculi) from which the hydrochloric acid is secreted into the lumen of the stomach. The pH level is maintained by the proton pumpH+/K+ ATPase. The parietal cell releases bicarbonate into the bloodstream in the process, which causes a temporary rise of pH in the blood, known as an alkaline tide.
The gastric juice also contains digestive enzymes produced by other cells in the gastric glands – gastric chief cells. Gastric chief cells secrete an inactivated pepsinogen. Once in the stomach lumen gastric acid activates the proenzyme to pepsin.
Secretion
A typical adult human stomach will secrete about 1.5 liters of gastric juice daily.[3] Gastric juice is the combination of gastric gland secretions including the main component of hydrochloric acid (gastric acid), gastric lipase and pepsinogen. Once in the stomach pepsinogen is changed by gastric acid to the digestive enzyme pepsin adding this enzyme to the gastric juice. In humans, the pH of gastric acid is between one and three, much lower than most other animals, but is very similar to that of carrion eating carnivores, needing extra protection from ingesting pathogens.
Gastric acid secretion is produced in several steps. Chloride and hydrogen ions are secreted separately from the cytoplasm of parietal cells and mixed in the canaliculi. This creates a negative potential of between −40and−70mV across the parietal cell membrane that causes potassium ions and a small number of sodium ions to diffuse from the cytoplasm into the parietal cell canaliculi. Gastric acid is then secreted along with other gland secretions into the gastric pit for release into the stomach lumen.
The enzyme carbonic anhydrase catalyses the reaction between carbon dioxide and water to form carbonic acid. This acid immediately dissociates into hydrogen and bicarbonate ions. The hydrogen ions leave the cell through H+/K+ ATPaseantiporter pumps.
At the same time, sodium ions are actively reabsorbed. This means that the majority of secreted K+ (potassium) and Na+
(sodium) ions return to the cytoplasm. In the canaliculus, secreted
hydrogen and chloride ions mix and are secreted into the lumen of the oxyntic gland.
The highest concentration that gastric acid reaches in the stomach is 160mM in the canaliculi. This is about 3 million times that of arterialblood, but almost exactly isotonic with other bodily fluids. The lowest pH of the secreted acid is 0.8, but the acid is diluted in the stomach lumen to a pH of between 1 and 3.
There is a small continuous basal secretion of gastric acid between meals of usually less than 10mEq/hour.
There are three phases in the secretion of gastric acid which increase the secretion rate in order to digest a meal:
The gastric phase:
About sixty percent of the total acid for a meal is secreted in this
phase. Acid secretion is stimulated by distension of the stomach and by amino acids present in the food.
The intestinal phase: The remaining 10% of acid is secreted when chyme enters the small intestine, and is stimulated by small intestine distension and by amino acids. The duodenal cells release entero-oxyntin which acts on parietal cells without affecting gastrin.
The production of gastric acid in the stomach is tightly regulated by positive regulators and negative feedback mechanisms. Four types of cells are involved in this process: parietal cells, G cells, D cells
and enterochromaffin-like cells. Beside this, the endings of the vagus
nerve (CN X) and the intramural nervous plexus in the digestive tract
influence the secretion significantly.
Nerve endings in the stomach secrete two stimulatory neurotransmitters: acetylcholine and gastrin-releasing peptide.
Their action is both direct on parietal cells and mediated through the
secretion of gastrin from G cells and histamine from
enterochromaffin-like cells. Gastrin acts on parietal cells directly and
indirectly too, by stimulating the release of histamine.
The release of histamine is the most important positive
regulation mechanism of the secretion of gastric acid in the stomach.
Its release is stimulated by gastrin and acetylcholine and inhibited by somatostatin.
Neutralization
In the duodenum, gastric acid is neutralized by bicarbonate. This also blocks gastric enzymes that have their optima in the acid range of pH. The secretion of bicarbonate from the pancreas is stimulated by secretin. This polypeptide hormone gets activated and secreted from so-called S cells in the mucosa of the duodenum and jejunum when the pH in the duodenum falls below 4.5 to 5.0. The neutralization is described by the equation:
HCl + NaHCO3 → NaCl + H2CO3
The carbonic acid rapidly equilibrates with carbon dioxide and water through catalysis by carbonic anhydrase enzymes bound to the gut epithelial lining,
leading to a net release of carbon dioxide gas within the lumen
associated with neutralisation. In the absorptive upper intestine, such
as the duodenum, both the dissolved carbon dioxide and carbonic acid
will tend to equilibrate with the blood, leading to most of the gas
produced on neutralisation being exhaled through the lungs.
Clinical significance
Gastroesophageal reflux disease (GERD) is a common disorder that occurs when stomach acid repeatedly flows back into the esophagus, this backwash of acid (reflux) also known as heartburn
can irritate the lining of the esophagus. Most people are able to
manage the discomfort of GERD with lifestyle changes and medications,
notably proton pump inhibitors, and H2 blockers. Antacids may also be used to neutralise gastric acid. Sometimes, surgery may be needed to ease symptoms.
Chronic inflammation of the gastric mucosa can lead to atrophic gastritis resulting in a decreased secretion of gastric acid, and consequent digestive problems.
This section needs expansion. You can help by adding to it. (November 2010)
The role of gastric acid in digestion was established in the 1820s and 1830s by William Beaumont on Alexis St. Martin, who, as a result of an accident, had a fistula
(hole) in his stomach, which allowed Beaumont to observe the process of
digestion and to extract gastric acid, verifying that acid played a
crucial role in digestion.
Proton-pump inhibitors (PPIs) are a class of medications that cause a profound and prolonged reduction of stomach acid production. They do so by irreversibly inhibiting the stomach's H+/K+ ATPaseproton pump.
The body eventually synthesizes new proton pumps to replace the
irreversibly inhibited ones, a process driven by normal cellular
turnover, which gradually restores acid production.
Proton-pump inhibitors have largely superseded the H2-receptor antagonists, a group of medications with similar effects but a different mode of action, and heavy use of antacids. A potassium-competitive acid blocker (PCAB) revaprazan was marketed in Korea as an alternative to a PPI. A newer PCAB vonoprazan
with a faster and longer lasting action than revaprazan, and PPIs has
been marketed in Japan (2013), Russia (2021), and the US (2023).
Specialty professional organizations recommend that people take the
lowest effective PPI dose to achieve the desired therapeutic result when
used to treat gastroesophageal reflux disease long-term. In the United States, the Food and Drug Administration
(FDA) has advised that over-the-counter PPIs, such as Prilosec OTC,
should be used no more than three 14-day treatment courses over one
year.
Despite their extensive use, the quality of the evidence
supporting their use in some of these conditions is variable. The
effectiveness of PPIs has not been demonstrated for every case. For
example, although they reduce the incidence of esophageal adenocarcinoma in Barrett's oesophagus, they do not change the length affected. In addition, research in the UK has suggested that PPIs are not effective at treating persistent throat symptoms.
Indications for stopping PPIs
PPIs
are often used longer than necessary. In about half of people who are
hospitalized or seen at a primary care clinic there is no documented
reason for their long-term use of PPIs.
Some researchers believe that, given the little evidence of long-term
effectiveness, the cost of the medication and the potential for harm
means that clinicians should consider stopping PPIs in many people.
Adverse effects
In
general, proton pump inhibitors are well tolerated, and the incidence
of short-term adverse effects is relatively low. The range and
occurrence of adverse effects are similar for all of the PPIs, though they have been reported more frequently with omeprazole. This may be due to its longer availability and, hence, clinical experience.
Long-term use of PPIs requires assessment of the balance of the benefits and risks of the therapy.
As of March 2017, various adverse outcomes have been associated with
long-term PPI use in several primary reports, but reviews assess the
overall quality of evidence in these studies as "low" or "very low". They describe inadequate evidence to establish causal relationships between PPI therapy and many of the proposed associations, due to study design and small estimates of effect size.
As of March 2017, benefits outweighed risks when PPIs are used
appropriately, but when used inappropriately, modest risks become
important.
They recommend that PPIs should be used at the lowest effective dose in
people with a proven indication, but discourage dose escalation and
continued chronic therapy in people unresponsive to initial empiric
therapy.
With regard to iron and vitamin B12, the data is weak and several confounding factors have been identified. Low levels of magnesium can be found in people on PPI therapy and these can be reversed when they are switched to H2-receptor antagonist medications.
Bone
High dose or long-term use of PPIs carries an increased risk of bone fractures which was not found with short-term, low dose use; the FDA included a warning regarding this on PPI drug labels in 2010.
In infants, acid suppression therapy is frequently prescribed to
treat symptomatic gastroesophageal reflux in otherwise healthy infants
(that is: without gastroesophageal reflux disease). A study from 2019
showed that PPI use alone and together with histamine H2-receptor
antagonists was associated with an increased bone fracture hazard, which
was amplified by days of use and earlier initiation of therapy. The reason is not clear; increased bone break down by osteoclasts has been suggested.
A recent 2024 study published in the Journal of Clinical Endocrinology & Metabolism found that chronic use of PPIs in men is linked to lower trabecular bone quality.
Specifically, PPI use was associated with reduced lumbar spine
trabecular bone score (TBS), as well as lower bone mineral density (BMD)
T-scores in the lumbar spine, total hip, and femoral neck. These findings suggest that long-term PPI use may negatively affect bone health in men.
Gastrointestinal
Some studies have shown a correlation between use of PPIs and Clostridioides difficile infection.
While the data are contradictory and controversial, the FDA had
sufficient concern to include a warning about this adverse effect on the
label of PPI medications. Concerns have also been raised about spontaneous bacterial peritonitis (SBP) in older people taking PPIs and in people with irritable bowel syndrome
taking PPIs; both types of infections arise in these populations due to
underlying conditions and it is not clear if this is a class effect of
PPIs. PPIs may predispose an individual to developing small intestinal bacterial overgrowth or fungal overgrowth.
In cirrhotic patients, large volume of ascites and reduced esophageal motility by varices can provoke GERD. Acidic irritation, in return, may induce the rupture of varices.
Therefore, PPIs are often routinely prescribed for cirrhotic patients
to treat GERD and prevent variceal bleeding. However, it has been
recently shown that long term use of PPIs in patients with cirrhosis
increases the risk of SBP and is associated with the development of
clinical decompensation and liver-related death during long-term
follow-up.
There is evidence that PPI use alters the composition of the bacterial populations inhabiting the gut, the gut microbiota.
Although the mechanisms by which PPIs cause these changes are yet to be
determined, they may have a role in the increased risk of bacterial
infections with PPI use. These infections can include Helicobacter pylori
due to this species not favouring an acid environment, leading to an
increased risk of ulcers and gastric cancer risk in genetically
susceptible patients.
PPI use in people who have received attempted H. pylori eradication may also be associated with an increased risk of gastric cancer. The validity and robustness of this finding, with the lack of causality, have led to this association being questioned.
It is recommended that long-term PPIs should be used judiciously after
considering individual's risk–benefit profile, particularly among those
with history of H. pylori infection, and that further, well-designed, prospective studies are needed.
Long-term use of PPIs is associated with the development of benign polyps from fundic glands (which is distinct from fundic gland polyposis); these polyps do not cause cancer and resolve when PPIs are discontinued. There is concern that use of PPIs may mask gastric cancers or other serious gastric problems.
PPI use has also been associated with the development of microscopic colitis.
Cardiovascular
Associations
of PPI use and cardiovascular events have also been widely studied but
clear conclusions have not been made as these relative risks are
confounded by other factors. PPIs are commonly used in people with cardiovascular disease for gastric protection when aspirin is given for its antiplatelet actions. An interaction between PPIs and the metabolism of the platelet inhibitor clopidogrel is known and this drug is also often used in people with cardiac disease.
There are associations with an increased risk of stroke, but this
appears to be more likely to occur in people who already have an
elevated risk.
One suggested mechanism for cardiovascular effects is because PPIs bind and inhibit dimethylargininase, the enzyme that degrades asymmetric dimethylarginine (ADMA), resulting in higher ADMA levels and a decrease in bioavailable nitric oxide.
Cancer
A
2022 umbrella review of 21 meta-analyses shows an association between
proton-pump inhibitor use and an increased risk of four types of cancer.
Other
Associations
have been shown between PPI use and an increased risk of pneumonia,
particularly in the 30 days after starting therapy, where it was found
to be 50% higher in community use.[63][64] Other very weak associations of PPI use have been found, such as with chronic kidney disease,[65][66][67][22][68][69]dementia and Hepatocellular carcinoma (HCC).
As of 2016, results were derived from observational studies, it
remained uncertain whether such associations were causal relationships.
Mechanism of action
The activation of PPIs
Proton pump inhibitors act by irreversibly blocking the hydrogen/potassiumadenosine triphosphataseenzyme system (the H+/K+ ATPase, or, more commonly, the gastric proton pump) of the gastric parietal cells. The proton pump is the terminal stage in gastric acid secretion, being directly responsible for secreting H+ ions into the gastric lumen, making it an ideal target for inhibiting acid secretion.
Because the H,K-ATPase is the final step of acid secretion, an
inhibitor of this enzyme is more effective than receptor antagonists in
suppressing gastric acid secretion.
All of these drugs inhibit the gastric H,K-ATPase by covalent binding,
so the duration of their effect is longer than expected from their
levels in the blood.
Targeting the terminal step in acid production, as well as the irreversible nature of the inhibition, results in a class of medications that are significantly more effective than H2 antagonists and reduce gastric acid secretion by up to 99%.
Decreasing the acid in the stomach can aid the healing of duodenal ulcers and reduce the pain from indigestion and heartburn. However, stomach acids are needed to digest proteins, vitamin B12, calcium, and other nutrients, and too little stomach acid causes the condition hypochlorhydria.
The PPIs are given in an inactive form, which is neutrally charged (lipophilic) and readily crosses cell membranes
into intracellular compartments (like the parietal cell canaliculus)
with acidic environments. In an acid environment, the inactive drug is
protonated and rearranges into its active form. As described above, the
active form will covalently and irreversibly bind to the gastric proton pump, deactivating it.
In H. pylori eradication,
PPIs help by increasing the stomach pH, causing the bacterium to shift
out of its coccoid form which is resistant to both acids and
antibiotics. PPIs also show some weaker additional effects in
eradication.
Pharmacokinetics
The rate of omeprazole absorption is decreased by concomitant food intake.
In addition, the absorption of lansoprazole and esomeprazole is
decreased and delayed by food. It has been reported, however, that these
pharmacokinetic effects have no significant impact on efficacy.
In healthy humans, the half-life of PPIs is about 1 hour (9 hours for tenatoprazole), but the duration of acid inhibition is 48 hours because of irreversible binding to the H,K-ATPase. All the PPIs except tenatoprazole are rapidly metabolized in the liver by CYP enzymes (mostly by CYP2C19 and 3A4). Dissociation of the inhibitory complex is probably due to the effect of the endogenousantioxidantglutathione which leads to the release of omeprazole sulfide and reactivation of the enzyme.
PPIs were developed in the 1980s, with omeprazole being launched in 1988. Most of these medications are benzimidazole derivatives, related to omeprazole, but imidazopyridine derivatives such as tenatoprazole have also been developed. Potassium-competitive inhibitors such as revaprazan reversibly block the potassium-binding site of the proton pump, acting more quickly, but are not available in most countries.
Society and culture
Economics
In British Columbia, Canada the cost of the PPIs varies significantly from CA$0.13 to CA$2.38 per dose while all agents in the class appear more or less equally effective.
Regulatory approval
A comparative table of FDA-approved indications for PPIs is shown below.
USS Growler,
one of two submarines designed to provide a nuclear deterrence using
cruise missiles with a 500-mile (800 km) range—placed on patrol by
starting to carry the Regulus I missile (shown at Pier 86 in New York, its home as a museum ship)
Deterrence theory refers to the scholarship and practice of
how threats of using force by one party can convince another party to
refrain from initiating some other course of action. The topic gained increased prominence as a military strategy during the Cold War with regard to the use of nuclear weapons and is related to but distinct from the concept of mutual assured destruction, according to which a full-scale nuclear attack on a power with second-strike capability
would devastate both parties. The central problem of deterrence
revolves around how to credibly threaten military action or nuclear
punishment on the adversary despite its costs to the deterrer. Deterrence in an international relations context is the application of deterrence theory to avoid conflict.
Deterrence is widely defined as any use of threats (implicit or explicit) or limited force intended to dissuade an actor from taking an action (i.e. maintain the status quo). Deterrence is unlike compellence, which is the attempt to get an actor (such as a state) to take an action (i.e. alter the status quo).Both are forms of coercion. Compellence has been characterized as harder to successfully implement than deterrence. Deterrence also tends to be distinguished from defense or the use of full force in wartime.
Deterrence is most likely to be successful when a prospective
attacker believes that the probability of success is low and the costs
of attack are high. Central problems of deterrence include the credible communication of threats and assurance. Deterrence does not necessarily require military superiority.
"General deterrence" is considered successful when an actor who might otherwise take an action refrains from doing so due to the consequences that the deterrer is perceived likely to take. "Immediate deterrence" is considered successful when an actor seriously contemplating immediate military force or action refrains from doing so. Scholars distinguish between "extended deterrence" (the protection of allies) and "direct deterrence" (protection of oneself). Rational deterrence theory holds that an attacker will be deterred if they believe that:
(Probability
of deterrer carrying out deterrent threat × Costs if threat carried
out) > (Probability of the attacker accomplishing the action ×
Benefits of the action)
This model is frequently simplified in game-theoretic terms as:
Costs × P(Costs) > Benefits × P(Benefits)
History
While
the concept of deterrence precedes the Cold War, it was during the Cold
War that the concept evolved into a clearly articulated objective in
strategic planning and diplomacy, with considerable analysis by
scholars.
By November 1945 general Curtis LeMay, who led American air raids on Japan during World War II, was thinking about how the next war would be fought. He said in a speech that month to the Ohio Society of New York that since "No air attack, once it is launched, can be completely stopped",
his country needed an air force that could immediately retaliate: "If
we are prepared it may never come. It is not immediately conceivable
that any nation will dare to attack us if we are prepared".
Most of the innovative work on deterrence theory occurred from the late 1940s to mid-1960s. Historically, scholarship on deterrence has tended to focus on nuclear deterrence.
Since the end of the Cold War, there has been an extension of
deterrence scholarship to areas that are not specifically about nuclear
weapons.
NATO was founded in 1949 with deterring aggression as one of its goals.
A distinction is sometimes made between nuclear deterrence and "conventional deterrence."
The two most prominent deterrent strategies are "denial" (denying
the attacker the benefits of attack) and "punishment" (inflicting costs
on the attacker).
Lesson of Munich, where appeasement failed, contributes to deterrence theory. In the words of scholars Frederik Logevall and Kenneth Osgood, "Munich and appeasement have become among the dirtiest words in American politics,
synonymous with naivete and weakness, and signifying a craven
willingness to barter away the nation's vital interests for empty
promises." They claimed that the success of US foreign policy
often depends upon a president withstanding "the inevitable charges of
appeasement that accompany any decision to negotiate with hostile
powers.
Concept
The
concept of deterrence can be defined as the use of threats in limited
force by one party to convince another party to refrain from initiating
some course of action. In Arms and Influence
(1966), Schelling offers a broader definition of deterrence, as he
defines it as "to prevent from action by fear of consequences."
Glenn Snyder also offers a broad definition of deterrence, as he argues
that deterrence involves both the threat of sanction and the promise of
reward.
A threat serves as a deterrent to the extent that it convinces
its target not to carry out the intended action because of the costs and
losses that target would incur. In international security, a policy of
deterrence generally refers to threats of military retaliation directed
by the leaders of one state to the leaders of another in an attempt to
prevent the other state from resorting to the use of military force in
pursuit of its foreign policy goals.
As outlined by Huth,
a policy of deterrence can fit into two broad categories: preventing an
armed attack against a state's own territory (known as direct
deterrence) or preventing an armed attack against another state (known
as extended deterrence). Situations of direct deterrence often occur if
there is a territorial dispute
between neighboring states in which major powers like the United States
do not directly intervene. On the other hand, situations of extended
deterrence often occur when a great power
becomes involved. The latter case has generated most interest in
academic literature. Building on the two broad categories, Huth goes on
to outline that deterrence policies may be implemented in response to a
pressing short-term threat (known as immediate deterrence) or as
strategy to prevent a military conflict or short-term threat from
arising (known as general deterrence).
A successful deterrence policy must be considered in military
terms but also political terms: International relations, foreign policy
and diplomacy. In military terms, deterrence success refers to
preventing state leaders from issuing military threats and actions that
escalate peacetime diplomatic and military co-operation into a crisis or
militarized confrontation that threatens armed conflict and possibly
war. The prevention of crises of wars, however, is not the only aim of
deterrence. In addition, defending states must be able to resist the
political and the military demands of a potential attacking nation. If
armed conflict is avoided at the price of diplomatic concessions to the
maximum demands of the potential attacking nation under the threat of
war, it cannot be claimed that deterrence has succeeded.
Furthermore, as Jentleson et al.
argue, two key sets of factors for successful deterrence are important:
a defending state strategy that balances credible coercion and deft
diplomacy consistent with the three criteria of proportionality,
reciprocity, and coercive credibility and minimizes international and
domestic constraints and the extent of an attacking state's
vulnerability as shaped by its domestic political and economic
conditions. In broad terms, a state wishing to implement a strategy of
deterrence is most likely to succeed if the costs of noncompliance that
it can impose on and the benefits of compliance it can offer to another
state are greater than the benefits of noncompliance and the costs of
compliance.
Deterrence theory holds that nuclear weapons are intended to
deter other states from attacking with their nuclear weapons, through
the promise of retaliation and possibly mutually assured destruction. Nuclear deterrence can also be applied to an attack by conventional forces. For example, the doctrine of massive retaliation threatened to launch US nuclear weapons in response to Soviet attacks.
A successful nuclear deterrent requires a country to preserve its
ability to retaliate by responding before its own weapons are destroyed
or ensuring a second-strike capability. A nuclear deterrent is sometimes composed of a nuclear triad, as in the case of the nuclear weapons owned by the United States, Russia, China and India. Other countries, such as the United Kingdom and France, have only sea-based and air-based nuclear weapons.
Proportionality
Jentleson et al. provides further detail in relation to those factors.
Proportionality refers to the relationship between the defending
state's scope and nature of the objectives being pursued and the
instruments available for use to pursue them. The more the defending
state demands of another state, the higher that state's costs of
compliance and the greater need for the defending state's strategy to
increase the costs of noncompliance and the benefits of compliance. That
is a challenge, as deterrence is by definition a strategy of limited
means. George (1991) goes on to explain that deterrence sometimes goes
beyond threats to the actual use of military force, but if force is
actually used, it must be limited and fall short of full-scale use to
succeed.
The main source of disproportionality is an objective that goes beyond policy change to regime change,
which has been seen in Libya, Iraq, and North Korea. There, defending
states have sought to change the leadership of a state and to policy
changes relating primarily to their nuclear weapons programs.
Reciprocity
Secondly, Jentleson et al.outlines that reciprocity involves an explicit understanding of linkage
between the defending state's carrots and the attacking state's
concessions. The balance lies in not offering too little, too late or
for too much in return and not offering too much, too soon, or for too
little return.
Coercive credibility
Finally,
coercive credibility requires that in addition to calculations about
costs and benefits of co-operation, the defending state convincingly
conveys to the attacking state that failure to co-operate has
consequences. Threats, uses of force, and other coercive instruments
such as economic sanctions
must be sufficiently credible to raise the attacking state's perceived
costs of noncompliance. A defending state having a superior military
capability or economic strength in itself is not enough to ensure
credibility. Indeed, all three elements of a balanced deterrence
strategy are more likely to be achieved if other major international
actors like the UN or NATO are supportive, and opposition within the defending state's domestic politics is limited.
The other important considerations outlined by Jentleson et al.
that must be taken into consideration is the domestic political and
economic conditions in the attacking state affecting its vulnerability
to deterrence policies and the attacking state's ability to compensate
unfavourable power balances. The first factor is whether internal
political support and regime security are better served by defiance, or
there are domestic political gains to be made from improving relations
with the defending state. The second factor is an economic calculation
of the costs that military force, sanctions, and other coercive
instruments can impose and the benefits that trade and other economic
incentives may carry. That is partly a function of the strength and
flexibility of the attacking state's domestic economy and its capacity
to absorb or counter the costs being imposed. The third factor is the
role of elites and other key domestic political figures within the
attacking state. To the extent that such actors' interests are
threatened with the defending state's demands, they act to prevent or
block the defending state's demands.
Rational deterrence theory
One
approach to theorizing about deterrence has entailed the use of
rational choice and game-theoretic models of decision making (see game theory). Rational deterrence theory entails:
Rationality: actors are rational
Unitary actor assumption: actors are understood as unitary
Dyads: interactions tend to be between dyads (or triads) of states
Strategic interactions: actors consider the choices of other actors
Deterrence theorists have consistently argued that deterrence success
is more likely if a defending state's deterrent threat is credible to
an attacking state. Huth
outlines that a threat is considered credible if the defending state
possesses both the military capabilities to inflict substantial costs on
an attacking state in an armed conflict, and the attacking state
believes that the defending state is resolved to use its available
military forces. Huth
goes on to explain the four key factors for consideration under
rational deterrence theory: the military balance, signaling and
bargaining power, reputations for resolve, interests at stake.
The American economist Thomas Schelling
brought his background in game theory to the subject of studying
international deterrence. Schelling's (1966) classic work on deterrence
presents the concept that military strategy can no longer be defined as
the science of military victory. Instead, it is argued that military
strategy was now equally, if not more, the art of coercion, intimidation
and deterrence.
Schelling says the capacity to harm another state is now used as a
motivating factor for other states to avoid it and influence another
state's behavior. To be coercive or deter another state, violence must
be anticipated and avoidable by accommodation. It can therefore be
summarized that the use of the power to hurt as bargaining power is the
foundation of deterrence theory and is most successful when it is held
in reserve.
In an article celebrating Schelling's Nobel Memorial Prize for Economics, Michael Kinsley, Washington Postop‑ed
columnist and one of Schelling's former students, anecdotally
summarizes Schelling's reorientation of game theory thus: "[Y]ou're
standing at the edge of a cliff, chained by the ankle to someone else.
You'll be released, and one of you will get a large prize, as soon as
the other gives in. How do you persuade the other guy to give in, when
the only method at your disposal—threatening to push him off the
cliff—would doom you both? Answer: You start dancing, closer and closer
to the edge. That way, you don't have to convince him that you would do
something totally irrational: plunge him and yourself off the cliff. You
just have to convince him that you are prepared to take a higher risk
than he is of accidentally falling off the cliff. If you can do that,
you win."
Military balance
Deterrence
is often directed against state leaders who have specific territorial
goals that they seek to attain either by seizing disputed territory in a
limited military attack or by occupying disputed territory after the
decisive defeat of the adversary's armed forces. In either case, the
strategic orientation of potential attacking states generally is for the
short term and is driven by concerns about military cost and
effectiveness. For successful deterrence, defending states need the
military capacity to respond quickly and strongly to a range of
contingencies. Deterrence often fails if either a defending state or an
attacking state underestimates or overestimates the other's ability to
undertake a particular course of action.
Signaling and bargaining power
The
central problem for a state that seeks to communicate a credible
deterrent threat by diplomatic or military actions is that all defending
states have an incentive to act as if they are determined to resist an
attack in the hope that the attacking state will back away from military
conflict with a seemingly resolved adversary. If all defending states
have such incentives, potential attacking states may discount statements
made by defending states along with any movement of military forces as
merely bluffs. In that regard, rational deterrence theorists have argued
that costly signals are required to communicate the credibility of a
defending state's resolve. Those are actions and statements that clearly
increase the risk of a military conflict and also increase the costs of
backing down from a deterrent threat. States that bluff are unwilling
to cross a certain threshold of threat and military action for fear of
committing themselves to an armed conflict.
There are three different arguments that have been developed in
relation to the role of reputations in influencing deterrence outcomes.
The first argument focuses on a defending state's past behavior in
international disputes and crises, which creates strong beliefs in a
potential attacking state about the defending state's expected behaviour
in future conflicts. The credibilities of a defending state's policies
are arguably linked over time, and reputations for resolve have a
powerful causal impact on an attacking state's decision whether to
challenge either general or immediate deterrence. The second approach
argues that reputations have a limited impact on deterrence outcomes
because the credibility of deterrence is heavily determined by the
specific configuration of military capabilities, interests at stake, and
political constraints faced by a defending state in a given situation
of attempted deterrence. The argument of that school of thought is that
potential attacking states are not likely to draw strong inferences
about a defending states resolve from prior conflicts because potential
attacking states do not believe that a defending state's past behaviour
is a reliable predictor of future behavior. The third approach is a
middle ground between the first two approaches and argues that potential
attacking states are likely to draw reputational inferences about
resolve from the past behaviour of defending states only under certain
conditions. The insight is the expectation that decisionmakers use only
certain types of information when drawing inferences about reputations,
and an attacking state updates and revises its beliefs when a defending
state's unanticipated behavior cannot be explained by case-specific
variables.
An example shows that the problem extends to the perception of
the third parties as well as main adversaries and underlies the way in
which attempts at deterrence can fail and even backfire if the
assumptions about the others' perceptions are incorrect.
Interests at stake
Although
costly signaling and bargaining power are more well established
arguments in rational deterrence theory, the interests of defending
states are not as well known. Attacking states may look beyond the
short-term bargaining tactics of a defending state and seek to determine
what interests are at stake for the defending state that would justify
the risks of a military conflict. The argument is that defending states
that have greater interests at stake in a dispute are more resolved to
use force and more willing to endure military losses to secure those
interests. Even less well-established arguments are the specific
interests that are more salient to state leaders such as military
interests and economic interests.
Furthermore, Huth
argues that both supporters and critics of rational deterrence theory
agree that an unfavorable assessment of the domestic and international
status quo by state leaders can undermine or severely test the success
of deterrence. In a rational choice approach, if the expected utility of
not using force is reduced by a declining status quo position,
deterrence failure is more likely since the alternative option of using
force becomes relatively more attractive.
Tripwires
International relations scholars Dan Reiter and Paul Poast have argued that so-called "tripwires" do not deter aggression.
Tripwires entail that small forces are deployed abroad with the
assumption that an attack on them will trigger a greater deployment of
forces.
Dan Altman has argued that tripwires do work to deter aggression,
citing the Western deployment of forces to Berlin in 1948–1949 to deter
Soviet aggression as a successful example.
A 2022 study by Brian Blankenship and Erik Lin-Greenberg found
that high-resolve, low-capability signals (such as tripwires) were not
viewed as more reassuring to allies than low-resolve, high-capability
alternatives (such as forces stationed offshore). Their study cast doubt
on the reassuring value of tripwires.
In 1966, Schelling
is prescriptive in outlining the impact of the development of nuclear
weapons in the analysis of military power and deterrence. In his
analysis, before the widespread use of assured second strike capability,
or immediate reprisal, in the form of SSBN submarines, Schelling argues that nuclear weapons
give nations the potential to destroy their enemies but also the rest
of humanity without drawing immediate reprisal because of the lack of a
conceivable defense system and the speed with which nuclear weapons can
be deployed. A nation's credible threat of such severe damage empowers
their deterrence policies and fuels political coercion and military
deadlock, which can produce proxy warfare.
According to Kenneth Waltz, there are three requirements for successful nuclear deterrence:
Part of a state's nuclear arsenal must appear to be able to survive an attack by the adversary and be used for a retaliatory second strike
The state must not respond to false alarms of a strike by the adversary
The stability–instability paradox
is a key concept in rational deterrence theory. It states that when two
countries each have nuclear weapons, the probability of a direct war
between them greatly decreases, but the probability of minor or indirect
conflicts between them increases.
This occurs because rational actors want to avoid nuclear wars, and
thus they neither start major conflicts nor allow minor conflicts to
escalate into major conflicts—thus making it safe to engage in minor
conflicts. For instance, during the Cold War the United States and the Soviet Union never engaged each other in warfare, but fought proxy wars in Korea, Vietnam, Angola, the Middle East, Nicaragua and Afghanistan and spent substantial amounts of money and manpower on gaining relative influence over the third world.
Bernard Brodie wrote in 1959 that a credible nuclear deterrent must be always ready. An extended nuclear deterrence guarantee is also called a nuclear umbrella.
Scholars have debated whether having a superior nuclear arsenal
provides a deterrent against other nuclear-armed states with smaller
arsenals. Matthew Kroenig has argued that states with nuclear
superiority are more likely to win nuclear crises, whereas Todd Sechser, Matthew Fuhrmann and David C. Logan have challenged this assertion.
A 2023 study found that a state with nuclear weapons is less likely to
be targeted by non-nuclear states, but that a state with nuclear weapons
is not less likely to target other nuclear states in low-level
conflict.
A 2022 study by Kyungwon Suh suggests that nuclear superiority may not
reduce the likelihood that nuclear opponents will initiate nuclear
crises.
Proponents of nuclear deterrence theory argue that newly
nuclear-armed states may pose a short- or medium-term risk, but that
"nuclear learning" occurs over time as states learn to live with new
nuclear-armed states.
Mark S. Bell and Nicholas L. Miller have however argued that there is a
weak theoretical and empirical basis for notions of "nuclear learning."
Stages of US policy of deterrence
The US policy of deterrence during the Cold War underwent significant variations.
Containment
The early stages of the Cold War were generally characterized by the containment of communism, an aggressive stance on behalf of the US especially on developing nations under its sphere of influence. The period was characterized by numerous proxy wars throughout most of the globe, particularly Africa, Asia, Central America, and South America. One notable conflict was the Korean War. George F. Kennan, who is taken to be the founder of this policy in his Long Telegram,
asserted that he never advocated military intervention, merely economic
support, and that his ideas were misinterpreted as espoused by the
general public.
Détente
With the US drawdown from Vietnam, the normalization of US relations with China, and the Sino-Soviet Split, the policy of containment was abandoned and a new policy of détente
was established, with peaceful co-existence was sought between the
United States and the Soviet Union. Although all of those factors
contributed to this shift, the most important factor was probably the
rough parity achieved in stockpiling nuclear weapons with the clear
capability of mutual assured destruction
(MAD). Therefore, the period of détente was characterized by a general
reduction in the tension between the Soviet Union and the United States
and a thawing of the Cold War, which lasted from the late 1960s until
the start of the 1980s. The doctrine of mutual nuclear deterrence then
characterized relations between the United States and the Soviet Union
and relations with Russia until the onset of the New Cold War in the early 2010s. Since then, relations have been less clear.
Reagan era
A third shift occurred with US President Ronald Reagan's
arms build-up during the 1980s. Reagan attempted to justify the policy
by concerns of growing Soviet influence in Latin America and the
post-1979 revolutionary government of Iran. Similar to the old policy of containment, the US funded several proxy wars, including support for Saddam Hussein of Iraq during the Iran–Iraq War, support for the mujahideen in Afghanistan,
who were fighting for independence from the Soviet Union, and several
anticommunist movements in Latin America such as the overthrow of the Sandinista government in Nicaragua. The funding of the Contras in Nicaragua led to the Iran-Contra Affair, while overt support led to a ruling from the International Court of Justice against the United States in Nicaragua v. United States.
The final expression of the full impact of deterrence during the cold war can be seen in the agreement between Reagan and Mikhail Gorbachev
in 1985. They "agreed that a nuclear war cannot be won and must never
be fought. Recognizing that any conflict between the USSR and the U.S.
could have catastrophic consequences, they emphasized the importance of
preventing any war between them, whether nuclear or conventional. They
will not seek to achieve military superiority.".
While the army was dealing with the breakup of the Soviet Union
and the spread of nuclear technology to other nations beyond the United
States and Russia, the concept of deterrence took on a broader
multinational dimension. The US policy on deterrence after the Cold War
was outlined in 1995 in the document called "Essentials of Post–Cold War Deterrence".
It explains that while relations with Russia continue to follow the
traditional characteristics of MAD, but the US policy of deterrence
towards nations with minor nuclear capabilities should ensure by threats
of immense retaliation (or even pre-emptive action)
not to threaten the United States, its interests, or allies. The
document explains that such threats must also be used to ensure that
nations without nuclear technology refrain from developing nuclear
weapons and that a universal ban precludes any nation from maintaining chemical or biological weapons.
The current tensions with Iran and North Korea over their nuclear
programs are caused partly by the continuation of the policy of
deterrence.
Post-Cold War period
By the beginning of the 2022 Russian invasion of Ukraine,
many western hawks expressed the view that deterrence worked in that
war but only in one way – in favor of Russia. Former US security
advisor, John Bolton,
said: Deterrence is working in the Ukraine crisis, just not for the
right side. The United States and its allies failed to deter Russia
from invading. The purpose of deterrence strategy is to prevent the
conflict entirely, and there Washington failed badly. On the other hand,
Russian deterrence is enjoying spectacular success. Russia has
convinced the West that even a whisper of NATO military action in
Ukraine would bring disastrous consequences. Putin threatens, blusters,
uses the word “nuclear,” and the West wilts.
When Elon Musk prevented Ukraine from carrying drone attacks on the Russian Black Sea fleet by denying to enable needed Starlink communications in Crimea, Anne Applebaum argued Musk had been deterred by Russia after the country's ambassador warned him an attack on Crimea would be met with a nuclear response.
Later Ukrainian attacks on the same fleet using a different
communications system also caused deterrence, this time to the Russian
Navy.
Timo S. Koster who served at NATO as Director of Defence Policy
& Capabilities similarly argued: A massacre is taking place in
Europe and the strongest military alliance in the world is staying out
of it. We are deterred and Russia is not. Philip Breedlove, a retired four-star U.S. Air Force general and a former SACEUR,
said that Western fears about nuclear weapons and World War III have
left it "fully deterred" and Putin "completely undeterred." The West
have "ceded the initiative to the enemy."
No attempt was made by NATO to deter Moscow with the threat of military
force, wondered another expert. To the contrary, it was Russia’s
deterrence that proved to be successful.
Since the early 2000s, there has been an increased focus on cyber deterrence. Cyber deterrence has two meanings:
The use of cyber actions to deter other states
The deterrence of an adversary's cyber operations
Scholars have debated how cyber capabilities alter traditional
understandings of deterrence, given that it may be harder to attribute
responsibility for cyber attacks, the barriers to entry may be lower,
the risks and costs may be lower for actors who conduct cyber attacks,
it may be harder to signal and interpret intentions, the advantage of
offense over defense, and weak actors and non-state actors can develop
considerable cyber capabilities.
Scholars have also debated the feasibility of launching highly damaging
cyber attacks and engaging in destructive cyber warfare, with most
scholars expressing skepticism that cyber capabilities have enhanced the
ability of states to launch highly destructive attacks. The most prominent cyber attack to date is the Stuxnet attack on Iran's nuclear program. By 2019, the only publicly acknowledged case of a cyber attack causing a power outage was the 2015 Ukraine power grid hack.
There are various ways to engage in cyber deterrence:
Denial: preventing adversaries from achieving military objectives by defending against them
Punishment: the imposition of costs on the adversary
Norms: the establishment and maintenance of norms that establish appropriate standards of behavior
Escalation: raising the probability that costs will be imposed on the adversary
Entanglement and interdependence: interdependence between actors can have a deterrent effect
There is a risk of unintended escalation in cyberspace due to difficulties in discerning the intent of attackers, and complexities in state-hacker relationships. According to political scientists Joseph Brown and Tanisha Fazal,
states frequently neither confirm nor deny responsibility for cyber
operations so that they can avoid the escalatory risks (that come with
public credit) while also signaling that they have cyber capabilities
and resolve (which can be achieved if intelligence agencies and
governments believe they were responsible).
According to Lennart Maschmeyer, cyber weapons have limited
coercive effectiveness due to a trilemma "whereby speed, intensity, and
control are negatively correlated. These constraints pose a trilemma for
actors because a gain in one variable tends to produce losses across
the other two variables."
Intrawar deterrence
Intrawar
deterrence is deterrence within a war context. It means that war has
broken out but actors still seek to deter certain forms of behavior. In
the words of Caitlin Talmadge, "intra-war deterrence failures... can be
thought of as causing wars to get worse in some way."
Examples of intrawar deterrence include deterring adversaries from
resorting to nuclear, chemical and biological weapons attacks or
attacking civilian populations indiscriminately. Broadly, it involves any prevention of escalation.
Latent nuclear deterrence
Some
scholars refer to the ability of some states to rapidly develop or gain
nuclear weapons as "latent nuclear deterrence". These states do not
necessarily aim to go all the way in building nuclear weapons, but they
may develop the civilian nuclear technology that would rapidly enable
them to build a nuclear weapon. They can use this "nuclear latency"
status for coercive purposes, as they can deter adversaries who do not
wish to see the state develop nuclear weapons or potentially use those
nuclear weapons.
Deterrence theory has been criticized by numerous scholars for
various reasons, the most basic being skepticism that decision makers
are rational. A prominent strain of criticism argues that rational
deterrence theory is contradicted by frequent deterrence failures, which
may be attributed to misperceptions. Here it's argued that misestimations of perceived costs and benefits by analysts contribute to deterrence failures, as exemplified in case of Russian invasion of Ukraine. Frozen conflicts can be seen as rewarding aggression.
Misprediction of behavior
Scholars
have also argued that leaders do not behave in ways that are consistent
with the predictions of nuclear deterrence theory.Scholars have also argued that rational deterrence theory does not
grapple sufficiently with emotions and psychological biases that make
accidents, loss of self-control, and loss of control over others likely.
Frank C. Zagare has argued that deterrence theory is logically
inconsistent and empirically inaccurate. In place of classical
deterrence, rational choice scholars have argued for perfect deterrence,
which assumes that states may vary in their internal characteristics
and especially in the credibility of their threats of retaliation.
Suicide attacks
Advocates for nuclear disarmament, such as Global Zero, have criticized nuclear deterrence theory. Sam Nunn, William Perry, Henry Kissinger, and George Shultz
have all called upon governments to embrace the vision of a world free
of nuclear weapons, and created the Nuclear Security Project to advance
that agenda. In 2010, the four were featured in a documentary film entitled Nuclear Tipping Point where proposed steps to achieve nuclear disarmament.
Kissinger has argued, "The classical notion of deterrence was that
there was some consequences before which aggressors and evildoers would
recoil. In a world of suicide bombers, that calculation doesn't operate
in any comparable way."
Shultz said, "If you think of the people who are doing suicide attacks,
and people like that get a nuclear weapon, they are almost by
definition not deterrable."
Stronger deterrent
Paul Nitze
argued in 1994 that nuclear weapons were obsolete in the "new world
disorder" after the dissolution of the Soviet Union, and he advocated
reliance on precision guided munitions to secure a permanent military
advantage over future adversaries.
Minimum deterrence
As opposed to the extreme mutually assured destruction form of deterrence, the concept of minimum deterrence
in which a state possesses no more nuclear weapons than is necessary to
deter an adversary from attacking is presently the most common form of
deterrence practiced by nuclear weapon states, such as China, India, Pakistan, Britain, and France. Pursuing minimal deterrence during arms negotiations between the United States and Russia allows each state to make nuclear stockpile
reductions without the state becoming vulnerable, but it has been noted
that there comes a point that further reductions may be undesirable,
once minimal deterrence is reached, as further reductions beyond that
point increase a state's vulnerability and provide an incentive for an
adversary to expand its nuclear arsenal secretly.
France has developed and maintained its own nuclear deterrent under the belief that the United States will refuse to risk its own cities by assisting Western Europe in a nuclear war.
Ethical objections
In the post cold war era, philosophical objections to the reliance upon deterrence theories in general have also been raised on purely ethical grounds. Scholars such as Robert L. Holmes have noted that the implementation of such theories is inconsistent with a fundamental deontological presumption which prohibits the killing of innocent life. Consequently, such theories are prima facie
immoral in nature. In addition, he observes that deterrence theories
serve to perpetuate a state of mutual assured destruction between
nations over time. Holmes further argues that it is therefore both
irrational and immoral to utilize a methodology for perpetuating
international peace which relies exclusively upon the continuous
development of new iterations of the very weapons which it is designed
to prohibit.
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