Just war theory is a tradition of moral and legal reasoning that attempts to determine when states may justly resort to armed force and how that force may justly be conducted. It seeks a middle path between two extremes: pacifism, which holds that all war is morally prohibited, and political realism, which holds that morality has no purchase in relations between states engaged in organized violence. Instead, just war thinking holds that war, though terrible, can under specific conditions be morally permissible and even required -- and that even in war, some actions remain morally impermissible regardless of military advantage.
The tradition has produced a rich body of moral argument refined over twenty-five centuries. It now exists simultaneously as philosophical theory, international law, military doctrine, and practical framework for commanders, policymakers, and citizens evaluating the use of force by their governments.
Historical Origins: Cicero, Augustine, and Aquinas
The tradition's roots in Western thought lie in classical antiquity. Cicero, in De Officiis (written 44 BCE), articulated conditions that distinguished just war (bellum iustum) from mere brigandage: wars must be publicly declared by legitimate authority, waged only after peaceful means have been exhausted, and conducted with restraint toward non-combatants. Cicero's framework reflected Roman legal practice as much as philosophical analysis, but it established the basic structure that later theorists would develop.
Augustine of Hippo (354-430 CE) gave just war thinking its first systematic theological grounding. Writing against a background of Roman imperial collapse and responding to the pacifist implications of early Christian teaching, Augustine argued that war could be a form of Christian charity: a just prince might rightly use force to protect the innocent, punish wrongdoers, and restore peace, even if the violence itself was tragic. Augustine's crucial contribution was the concept of right intention: war fought from hatred, vengeance, or lust for domination was sinful even if its external cause was just. Only war fought with love -- a love that could express itself in coercive force directed at the reformation of wrongdoers -- could be just.
Thomas Aquinas synthesized these elements in the Summa Theologica (1265-1274), identifying three conditions: just authority (only sovereigns, not private individuals, can declare war), just cause (the enemy must have committed a wrong meriting punishment), and right intention (the goal must be to advance good and avoid evil, not to satisfy aggression or greed). Aquinas's formulation became the standard reference for subsequent Catholic moral theology and, through its influence on Vitoria and Grotius, for secular international law.
Francisco de Vitoria (1492-1546) extended the tradition in his lectures on the justice of wars against the indigenous peoples of the Americas, questioning whether conquest and conversion could constitute just cause. Hugo Grotius (1583-1645), often called the father of international law, developed just war principles into a secular legal framework in De Jure Belli ac Pacis (1625), arguing that these principles bound all nations by natural reason, not merely by Christian theology.
Jus Ad Bellum: When Is It Just to Go to War?
The jus ad bellum (justice of going to war) tradition identifies a set of criteria that must be satisfied for a decision to resort to armed force to be morally permissible. These criteria have been refined over centuries and are now embodied in different forms in international law, particularly the UN Charter's prohibition on aggressive war and its exceptions for self-defense and Security Council-authorized force.
Classical formulations typically include six interconnected criteria:
| Criterion | Core Requirement | Contemporary Application |
|---|---|---|
| Just cause | Response to genuine wrong (typically aggression) | Self-defense; Security Council authorization |
| Right intention | Genuine aim to achieve just cause, not pretext | Government's stated aims must be actual aims |
| Legitimate authority | War declared by sovereign political community | Elected governments; UN Charter framework |
| Last resort | Armed force only after other means genuinely tried | Diplomacy, sanctions, mediation exhausted |
| Proportionality of ends | Military benefits proportionate to anticipated harms | Cost-benefit analysis at strategic level |
| Reasonable chance of success | Realistic prospect of achieving just aims | Prohibits war with certain defeat |
Just cause requires that the resort to war be in response to a genuine wrong -- most standardly, armed aggression. Contemporary just war theorists debate whether preventive war against a gathering threat can satisfy just cause, or whether this standard applies only to actual or imminent attack. The distinction between preventive war (against a future threat) and preemptive war (against an imminent attack) is central to the controversy over the 2003 Iraq invasion, which many analysts concluded failed the just cause criterion.
Reasonable chance of success is the criterion that has most troubled theorists. It holds that a war with no realistic prospect of achieving its just aims should not be fought, since the costs of certain defeat without compensating achievement cannot be justified. Critics argue this penalizes the weak and potentially obligates passive acceptance of injustice. Finland's decision to fight the Soviet Union in the Winter War of 1939-1940, despite the obvious disparity in power, is often cited as a case where this criterion, strictly applied, might have counseled against a war that was nevertheless morally and practically defensible.
Jus In Bello: How Must War Be Conducted?
Jus in bello (justice in war) governs the conduct of armed forces once war has begun, regardless of whether the resort to war was itself justified. The two foundational principles are discrimination (or distinction) and proportionality in conduct -- principles now codified in international humanitarian law, particularly the Geneva Conventions and their Additional Protocols.
The principle of discrimination requires that combatants distinguish at all times between those who may be attacked -- enemy combatants and others taking direct part in hostilities -- and those who may not -- civilians and other non-combatants, including wounded soldiers, prisoners of war, and medical personnel. This is the principle of non-combatant immunity. It does not prohibit all civilian casualties, which are often an inevitable consequence of legitimate military operations, but it absolutely prohibits the intentional targeting of civilians.
The doctrine of double effect, with roots in Aquinas, is deployed to address the moral status of foreseeable but unintended civilian casualties. An act that produces both a good military effect (destroying an ammunition depot) and a bad civilian effect (killing nearby residents) can be permissible if:
- The act itself is not intrinsically wrong
- The agent intends only the good effect and not the bad one
- The bad effect is not the means by which the good effect is achieved
- The good effect is proportionate to the bad
Proportionality in conduct requires that the collateral harm to civilian life and property not be excessive in relation to the anticipated concrete military advantage. It is a context-sensitive rather than absolute standard, requiring commanders to make judgments under conditions of uncertainty and time pressure.
The principle of military necessity permits measures that are indispensable for achieving a legitimate military objective and not otherwise prohibited by international humanitarian law. It limits but does not eliminate constraints: necessity cannot justify acts that are absolutely prohibited -- torture, summary execution of prisoners, starvation of civilians as a method of warfare.
"War is never just a relation between states; it is always a relation between people, and people remain moral agents whatever their political condition." -- Michael Walzer, Just and Unjust Wars (1977)
Michael Walzer and the Supreme Emergency Exception
Michael Walzer's Just and Unjust Wars (1977) remains the most widely read and debated work in contemporary just war theory. Writing against the background of Vietnam-era debates, Walzer sought to rehabilitate moral reasoning about war against both utilitarian consequentialism and political realism. His approach was grounded in ordinary moral consciousness rather than abstract philosophical principle -- he began from the shared moral intuitions that ordinary people deploy when thinking about war.
Walzer's central contribution was his articulation of the war convention as a coherent set of moral rules governing military conduct, and his rigorous application of those rules to specific cases including the Nuremberg trials, the 1967 Six Day War, the Korean War, and My Lai. His case-based method made the book both accessible and analytically powerful.
The most controversial element of Walzer's theory is his supreme emergency exception. Walzer argues that in situations where a community faces "unusual and very great danger" -- specifically, the prospect of political annihilation at the hands of an enemy whose victory would represent a supreme evil -- the normal rules of war may temporarily be suspended. His paradigm case is Britain in 1940-41: facing potential conquest by Nazi Germany, when Britain stood alone and the threat to civilization was genuine and imminent, Walzer argues that Churchill's decision to target German cities (the area bombing campaign) might be justified as a desperate measure to buy time, despite its deliberate killing of civilians.
The exception is extremely narrow in Walzer's formulation -- it applies only when the threat is both catastrophically extreme and genuinely imminent -- and he insists that even when invoked, it does not eliminate guilt. Those who order civilian targeting under supreme emergency circumstances retain what Walzer calls dirty hands: they act in a way that saves the community but violates moral rules, and they remain morally tainted by the violation even if the action was necessary. Critics argue the exception is too permissive and too easily abused by governments eager to claim emergency status.
Humanitarian Intervention and the Responsibility to Protect
Humanitarian intervention -- the use of military force by outside powers to protect populations from severe human rights abuses by their own governments -- poses one of the most acute dilemmas in just war theory. It pits the foundational principle of state sovereignty and non-intervention against the imperative to protect individuals from mass atrocities.
NATO's 1999 intervention in Kosovo -- launched without Security Council authorization after Russia indicated it would veto approval -- halted Serbian ethnic cleansing of Albanians and established a precedent for humanitarian military action outside the UN Charter framework. The Independent International Commission on Kosovo's report coined the formulation "illegal but legitimate" to describe the gap between positive law and moral justification.
The 2003 invasion of Iraq generated a contrasting case. The US and UK governments advanced humanitarian arguments -- Saddam Hussein's atrocities against Kurds and Shia populations -- alongside dubious weapons of mass destruction claims. The intervention lacked Security Council authorization, was not supported by the broad multilateral coalition that had acted in Kosovo, and was pursued over explicit objections from France, Germany, Russia, and China. Most just war theorists concluded it failed multiple jus ad bellum criteria including last resort and proportionality.
The Responsibility to Protect (R2P) doctrine, endorsed at the 2005 World Summit, sought to provide a framework for humanitarian intervention that preserves the presumption of sovereignty while establishing conditions under which that sovereignty can be overridden. R2P holds that sovereignty entails responsibility: states have primary responsibility to protect their populations from genocide, war crimes, ethnic cleansing, and crimes against humanity, and when they fail to do so, the international community bears a responsibility to respond.
Libya in 2011 was the first case of Security Council authorization explicitly invoking R2P, though the NATO military operation's scope -- extending to regime change -- generated controversy that has inhibited subsequent R2P invocations. The major humanitarian catastrophes of the 2010s and 2020s -- Syria, Yemen, Myanmar, Sudan -- have occurred largely without effective international intervention, exposing the gap between R2P as doctrine and R2P as operative constraint on state behavior.
Drone Warfare and Targeted Killing
The development of remotely piloted aircraft systems and targeted killing programs has created significant new challenges for just war theory. The United States began using armed drones for targeted killing operations in Afghanistan in 2001 and extended the program to Yemen, Pakistan, Somalia, and other countries in subsequent years.
The most fundamental jus ad bellum question concerns the geographic and temporal scope of armed conflict. The US government's position holds that it is engaged in a non-international armed conflict with al-Qaeda and affiliated forces that is not geographically limited to specific battlefields where declared hostilities are occurring. This authorizes strikes in countries with which the United States is not at war. Critics argue this amounts to an unlimited license for extraterritorial killing whenever the government designates a target as a threat.
The jus in bello questions center on the principle of distinction and the definition of targetable individuals. The category of "unprivileged belligerent" or unlawful combatant, applied to members of non-state armed groups, is contested: does mere membership in al-Qaeda or its affiliates make an individual targetable at any time and place, or does direct participation in specific hostile acts determine targetability?
"Signature strikes" -- strikes based on patterns of behavior rather than positive identification of a specific known individual -- are particularly controversial. They require the intelligence community to make probabilistic judgments about targetability without the individual-level identification that the law of armed conflict's distinction principle seems to require.
Just war theorists including Mary Ellen O'Connell, Kenneth Roth, and Philip Alston have argued that targeted killing programs, particularly outside active combat zones, violate international law and just war principles.
Nuclear Deterrence and Its Moral Paradoxes
Nuclear deterrence presents just war theory with what many regard as its most intractable problem. Deterrence works by maintaining a credible threat to inflict catastrophic harm -- potentially on millions of civilians -- on any state that launches a nuclear attack. Its moral structure depends on two elements: the genuine capacity to retaliate with nuclear weapons, and a genuine intention to use that capacity under specified circumstances.
But if the intention to retaliate is genuine, deterrence requires maintaining an actual commitment to engage in what would be a clear violation of just war principles -- the mass killing of civilians. The International Court of Justice's 1996 advisory opinion on the legality of nuclear weapons found that the threat or use of nuclear weapons would generally be contrary to international humanitarian law, though the court declined to render a definitive ruling on whether such use could ever be legal in extreme circumstances of self-defense.
Defenders of nuclear deterrence have offered several just war responses:
- Deterrence as a systemic policy is morally different from any individual decision to fire; the moral evaluation should apply to the policy level rather than the individual level
- The deterrent is a conditional intention -- conditional on the adversary refraining from attack -- and the condition need never be fulfilled
- Abandoning deterrence carries its own catastrophic risks, and the consequence of abandonment may be worse than maintaining a morally problematic deterrent
The 1983 US Catholic Bishops' pastoral letter The Challenge of Peace took an influential middle position: deterrence was acceptable as a transitional arrangement provided it was accompanied by genuine pursuit of disarmament. The letter condemned any nuclear use that would directly target civilian populations and conditioned acceptance of deterrence on active movement toward arms reduction -- a condition that subsequent decades have not consistently satisfied.
Practical Significance: Why Just War Theory Matters
Just war theory might seem like an academic exercise remote from the realities of military conflict. In fact, it has substantial practical effects through several channels.
Military training and doctrine incorporate just war principles directly. International humanitarian law -- itself codified just war theory -- is taught to military officers worldwide, and serious professional militaries maintain legal review processes for targeting decisions, rules of engagement, and interrogation practices. The accountability tribunals that followed World War II, the former Yugoslavia, and Rwanda applied just war-derived principles to prosecute individuals for war crimes and crimes against humanity.
Policy legitimacy depends on just war framing. Governments that go to war must justify their actions in terms recognizable to international audiences, domestic publics, and allied governments. Even states with no genuine commitment to just war constraints often argue in just war terms because the framework shapes the standards against which they are judged.
Moral clarity for citizens is perhaps the most important practical function. Citizens in democratic societies are responsible for the wars their governments fight on their behalf. Just war theory provides the conceptual vocabulary for evaluating whether those wars are justified, whether they are being conducted acceptably, and when public opposition is morally warranted. The framework does not resolve all difficult cases -- it provides principled arguments and counterarguments -- but it enables more sophisticated moral evaluation than the alternatives of unconditional deference to governments or unconditional pacifism.
The tradition's endurance across twenty-five centuries, through technological transformations that have repeatedly altered the scale and character of warfare, testifies to the genuine moral problems it addresses. As long as states have both the capacity to wage war and the need to justify it, just war thinking will remain indispensable.
Frequently Asked Questions
What is just war theory and what are its historical origins?
Just war theory is a tradition of moral and legal reasoning that attempts to determine when states may justly resort to armed force and how that force may justly be conducted. It seeks a middle path between pacifism - which holds that all war is morally prohibited - and political realism - which holds that morality has no purchase in relations between states engaged in organized violence. Instead, just war thinking holds that war, though terrible, can under specific conditions be morally permissible and even required, and that even in war, some actions remain morally impermissible regardless of military advantage.The tradition's roots in Western thought lie in classical antiquity. Cicero, in De Officiis (written 44 BCE), articulated conditions that distinguished just war (bellum iustum) from mere brigandage: wars must be publicly declared by legitimate authority, waged only after peaceful means have been exhausted, and conducted with restraint toward non-combatants. Cicero's framework reflected Roman legal practice as much as philosophical analysis, but it established the basic structure that later theorists would develop.Augustine of Hippo (354-430 CE) gave just war thinking its first systematic theological grounding. Writing against a background of Roman imperial collapse and responding to the pacifist implications of early Christian teaching, Augustine argued that war could be a form of Christian charity: a just prince might rightly use force to protect the innocent, punish wrongdoers, and restore peace, even if the violence itself was tragic. Augustine's crucial contribution was the concept of right intention: war fought from hatred, vengeance, or lust for domination was sinful even if its external cause was just. Only war fought with love - a love that could express itself in coercive force directed at the reformation of wrongdoers - could be just.Thomas Aquinas synthesized these elements in the Summa Theologica (1265-1274), identifying three conditions: just authority (only sovereigns, not private individuals, can declare war), just cause (the enemy must have committed a wrong meriting punishment), and right intention (the goal must be to advance good and avoid evil, not to satisfy aggression or greed). Aquinas's formulation became the standard reference for subsequent Catholic moral theology and, through its influence on Vitoria and Grotius, for secular international law.
What are the jus ad bellum criteria that determine whether going to war is justified?
The jus ad bellum (justice of going to war) tradition identifies a set of criteria that must be satisfied for a decision to resort to armed force to be morally permissible. These criteria have been refined over centuries and are now embodied in different forms in international law, particularly the UN Charter's prohibition on aggressive war and its exceptions for self-defense and Security Council-authorized force. Classical formulations typically include six interconnected criteria.Just cause requires that the resort to war be in response to a genuine wrong - most standardly, armed aggression, though the tradition has also recognized defense of the innocent, recovery of goods wrongfully taken, and punishment of manifest injustice. Contemporary just war theorists debate whether preventive war against a gathering threat can satisfy just cause, or whether this standard applies only to actual or imminent attack.Right intention holds that even where just cause exists, the war must be fought with the aim of achieving the just cause rather than using it as a pretext for aggression, conquest, or revenge. Nations may have just cause to respond to aggression while harboring unjust intentions; the theory requires the stated cause to be the genuine motivation.Legitimate authority holds that war may only be declared and conducted by those with proper authority to do so - traditionally, sovereign political communities rather than private armed groups. Contemporary applications raise questions about whether non-state actors can ever satisfy this criterion and about the legitimacy of Security Council authorization.Last resort requires that armed force be employed only after other means - diplomacy, sanctions, mediation, arbitration - have been genuinely attempted and failed. This criterion rarely requires that literally every alternative be exhausted to the point of impossibility, but it does demand serious good-faith effort to resolve the dispute peacefully.Proportionality of ends requires that the anticipated military and political benefits of the war be proportionate to the harms anticipated from waging it. A war fought to recover a minor territorial wrong but expected to produce mass casualties would fail this test.Reasonable chance of success is the criterion that has most troubled theorists. It holds that a war with no realistic prospect of achieving its just aims should not be fought, since the costs of certain defeat without compensating achievement cannot be justified. Critics argue this penalizes the weak and potentially obligates passive acceptance of injustice.
What are the jus in bello rules that govern how war is conducted?
Jus in bello (justice in war) governs the conduct of armed forces once war has begun, regardless of whether the resort to war was itself justified. The two foundational principles are discrimination (or distinction) and proportionality in conduct - principles that are now codified in international humanitarian law, particularly the Geneva Conventions and their Additional Protocols.The principle of discrimination requires that combatants distinguish at all times between those who may be attacked - enemy combatants and others taking direct part in hostilities - and those who may not - civilians and other non-combatants, including wounded soldiers, prisoners of war, and medical personnel. This is the principle of non-combatant immunity. It does not prohibit all civilian casualties, which are often an inevitable consequence of legitimate military operations, but it absolutely prohibits the intentional targeting of civilians and requires that military operations be directed only at legitimate military targets.The doctrine of double effect, with roots in Aquinas, is deployed to address the moral status of foreseeable but unintended civilian casualties. An act that produces both a good military effect (destroying an ammunition depot) and a bad civilian effect (killing nearby residents) can be permissible if: the act itself is not intrinsically wrong, the agent intends only the good effect and not the bad one, the bad effect is not the means by which the good effect is achieved, and the good effect is proportionate to the bad. This doctrine is both practically important and philosophically contested, particularly when applied to high-altitude bombing, drone strikes, or siege warfare.Propionality in conduct requires that the collateral harm to civilian life and property not be excessive in relation to the anticipated concrete military advantage. It is a context-sensitive rather than absolute standard, requiring commanders to make judgments under conditions of uncertainty and time pressure. Military manuals and international law tribunals have struggled to give this standard precise content.The principle of military necessity permits measures that are indispensable for achieving a legitimate military objective and not otherwise prohibited by international humanitarian law. It limits but does not eliminate constraints: necessity cannot justify acts that are absolutely prohibited - torture, summary execution of prisoners, starvation of civilians as a method of warfare.
What is Michael Walzer's contribution to just war theory and what is the supreme emergency exception?
Michael Walzer's Just and Unjust Wars (1977) remains the most widely read and debated work in contemporary just war theory. Writing against the background of Vietnam-era debates about the ethics of military action, Walzer sought to rehabilitate moral reasoning about war against both utilitarian consequentialism and political realism. His approach was grounded in ordinary moral consciousness rather than abstract philosophical principle: he began from the shared moral intuitions that ordinary people deploy when thinking about war - intuitions that reflect centuries of moral experience.Walzer's central contribution was his articulation of the war convention as a coherent set of moral rules governing military conduct, and his rigorous application of those rules to specific cases including the Nuremberg trials, the 1967 Six Day War, the Korean War, and My Lai. His case-based method made the book both accessible and analytically powerful, showing how general principles must be applied in particular circumstances and where that application generates genuine moral difficulty.The most controversial element of Walzer's theory is his 'supreme emergency' exception. Walzer argues that in situations where a community faces what he calls 'unusual and very great danger' - specifically, the prospect of political annihilation at the hands of an enemy whose victory would represent a supreme evil - the normal rules of war may temporarily be suspended. His paradigm case is Britain in 1940-41: facing potential conquest by Nazi Germany, when Britain stood alone and the threat to civilization was genuine and imminent, Walzer argues that Churchill's decision to target German cities (the area bombing campaign) might be justified as a desperate measure to buy time, despite its deliberate killing of civilians.The exception is extremely narrow in Walzer's formulation - it applies only when the threat is both catastrophically extreme and genuinely imminent - and he insists that even when invoked, it does not eliminate guilt. Those who order civilian targeting under supreme emergency circumstances retain what Walzer calls dirty hands: they act in a way that saves the community but violates moral rules, and they remain morally tainted by the violation even if the action was necessary. Critics argue the exception is too permissive and too easily abused by governments eager to claim emergency status.
When is humanitarian intervention morally justified and what are the main objections?
Humanitarian intervention - the use of military force by outside powers to protect populations from severe human rights abuses by their own governments - poses one of the most acute dilemmas in just war theory and international law. It pits the foundational principle of state sovereignty and non-intervention against the imperative to protect individuals from mass atrocities. The post-Cold War era produced several cases that sharpened this dilemma, leading to both the expansion of intervention practice and the development of the Responsibility to Protect doctrine.NATO's 1999 intervention in Kosovo - launched without Security Council authorization after Russia indicated it would veto approval - halted Serbian ethnic cleansing of Albanians in Kosovo and established a precedent for humanitarian military action outside the UN Charter framework. The intervention was widely regarded as illegal under existing international law but legitimate in the moral sense: the humanitarian necessity was real, other means had failed, and the intervention was proportionate and limited in scope. Independent International Commission on Kosovo's report coined the formulation 'illegal but legitimate' to describe the gap between positive law and moral justification.The 2003 invasion of Iraq generated a contrasting case. The US and UK governments advanced humanitarian arguments - Saddam Hussein's atrocities against Kurds and Shia populations - alongside dubious weapons of mass destruction claims. The intervention lacked Security Council authorization, was not supported by the broad multilateral coalition that had acted in Kosovo, and was pursued over explicit objections from France, Germany, Russia, and China. Most just war theorists concluded it failed multiple jus ad bellum criteria including last resort and proportionality, and that humanitarian justifications were post-hoc rationalizations for a war driven by other motives.The Responsibility to Protect (R2P) doctrine, endorsed at the 2005 World Summit, sought to provide a framework for humanitarian intervention that preserves the presumption of sovereignty while establishing conditions under which that sovereignty can be overridden. R2P holds that sovereignty entails responsibility: states have primary responsibility to protect their populations from genocide, war crimes, ethnic cleansing, and crimes against humanity, and when they fail to do so, the international community bears a responsibility to respond through diplomatic, humanitarian, and ultimately coercive means. Libya in 2011 was the first case of Security Council authorization explicitly invoking R2P, though the NATO military operation's scope - extending to regime change - generated controversy that has inhibited subsequent R2P invocations.
How does just war theory apply to drone warfare and targeted killing?
The development of remotely piloted aircraft systems (drones) and targeted killing programs has created significant new challenges for just war theory and international humanitarian law. The United States began using armed drones for targeted killing operations in Afghanistan in 2001 and extended the program to Yemen, Pakistan, Somalia, and other countries in subsequent years. Similar programs have been pursued by Israel, the United Kingdom, and other states. The technology raises both familiar and novel just war questions.The most fundamental jus ad bellum question concerns the geographic and temporal scope of armed conflict. The US government's position, developed in the years after September 11, holds that it is engaged in a non-international armed conflict with al-Qaeda and affiliated forces that is not geographically limited to specific battlefields where declared hostilities are occurring. This position authorizes strikes in countries with which the United States is not at war, such as Pakistan and Yemen, against individuals who are present in those countries. Critics argue this amounts to an unlimited license for extraterritorial killing whenever the government designates a target as a threat, without the spatial and temporal constraints that limit armed conflict in international law.The jus in bello questions center on the principle of distinction and the definition of targetable individuals. International humanitarian law permits targeting enemy combatants but prohibits targeting civilians except when they directly participate in hostilities. The category of 'unprivileged belligerent' or unlawful combatant, applied to members of non-state armed groups, is contested: does mere membership in al-Qaeda or its affiliates make an individual targetable at any time and place, or does direct participation in specific hostile acts determine targetability?Additional concerns include the reliability of intelligence used to identify targets (particularly 'signature strikes' based on patterns of behavior rather than positive identification), the standard for civilian casualty estimates and whether official figures are accurate, accountability mechanisms when civilians are killed, and the psychological effects of operating in areas where drone presence creates constant fear among civilian populations. Just war theorists including Mary Ellen O'Connell, Kenneth Roth, and Philip Alston have argued that targeted killing programs, particularly outside active combat zones, violate international law and just war principles.
How does just war theory address nuclear deterrence and its moral paradoxes?
Nuclear deterrence presents just war theory with what many regard as its most intractable problem. Deterrence works by maintaining a credible threat to inflict catastrophic harm - potentially on millions of civilians - on any state that launches a nuclear attack. Its moral structure depends on two elements: the genuine capacity to retaliate with nuclear weapons, and a genuine intention to use that capacity under specified circumstances. If the intention to retaliate is merely pretended, deterrence fails because a sophisticated adversary will eventually recognize the bluff. But if the intention is genuine, deterrence requires maintaining an actual commitment to engage in what would be a clear violation of just war principles - the mass killing of civilians.The just war tradition's principle of non-combatant immunity appears to prohibit both the use and the threatened use of nuclear weapons against civilian populations. The International Court of Justice's 1996 advisory opinion on the legality of nuclear weapons found that the threat or use of nuclear weapons would generally be contrary to international humanitarian law, though the court declined to render a definitive ruling on whether such use could ever be legal in extreme circumstances of self-defense.Defenders of nuclear deterrence have offered several just war responses. Some argue that deterrence itself is morally different from the intention to use: maintaining a deterrent structure is a systemic policy distinct from any individual decision to fire, and the moral evaluation should apply to the policy level rather than the individual level. Others distinguish conditional from unconditional intentions: the deterrent is conditional on the adversary refraining from attack, and the condition need never be fulfilled. Some argue that the consequence of abandoning deterrence - increased risk of conventional or nuclear war - is worse than maintaining a morally problematic deterrent.The 1983 US Catholic Bishops' pastoral letter The Challenge of Peace took an influential middle position: deterrence was acceptable as a transitional arrangement provided it was accompanied by genuine pursuit of disarmament, not merely as a permanent condition. The letter condemned any nuclear use that would directly target civilian populations and conditioned acceptance of deterrence on active movement toward arms reduction - a condition that subsequent decades have not consistently satisfied.