Just War theory has long provided a framework through which political leaders, theologians, and scholars assess the moral legitimacy of armed conflict. Developed within the Christian intellectual tradition through the writings of Augustine of Hippo and later systematised by Thomas Aquinas, the theory seeks to reconcile the moral prohibition against violence with the reality that war may sometimes be necessary to restrain injustice. In contemporary analysis the tradition is normally divided into two categories: jus ad bellum, which governs the justice of entering war, and jus in bello, which governs the conduct of war once fighting has begun. Applying these criteria to the recent United States attack on Iran, and comparing the case with the Iraq War and the Six-Day War, highlights the enduring tensions between security, legality, and moral justification in modern warfare.

Damage on Iran military site

The first requirement of jus ad bellum is just cause, most commonly understood as defence against aggression or imminent threat. The United States has justified military action against Iran primarily on the grounds of preventing future threats, particularly those associated with nuclear capability and regional destabilisation. However, Just War theory traditionally requires evidence of an immediate or clearly impending attack. Without such evidence, military action risks being classified as preventive war, which classical Just War theorists generally regard as morally suspect. In this respect, the American attack on Iran bears similarities to the rationale used in the Iraq War of 2003. The U.S. administration at that time argued that the regime of Saddam Hussein possessed weapons of mass destruction that posed a serious threat. Yet the absence of such weapons following the invasion undermined the claim of just cause and led many scholars to judge the war as inconsistent with traditional Just War criteria. By contrast, the Israeli decision to strike first in the Six-Day War was widely interpreted by many historians as a response to immediate and credible threats. Egyptian troop mobilisation in Sinai, the closure of the Straits of Tiran, and explicit statements by regional leaders created a perception of imminent conflict. Although debate continues among historians, many analysts view Israel’s pre-emptive strike as closer to a legitimate act of anticipatory self-defence than the American actions in either Iraq or Iran.

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The second key criterion is legitimate authority, meaning that war must be declared by a recognised political authority acting within legal bounds. In democratic systems this requirement often raises questions regarding constitutional processes. In the case of the American attack on Iran, critics argue that the absence of formal congressional authorisation raises doubts about the legitimacy of the decision under both domestic constitutional law and international norms. Similar debates occurred during the Iraq War, where the U.S. Congress authorised the use of force but the invasion lacked explicit approval from the United Nations Security Council, thereby weakening claims of international legitimacy. The Six-Day War, however, occurred within a different strategic and legal environment. Israel’s government acted as the recognised authority of the state in response to what it perceived as an immediate military threat from neighbouring states, a context that strengthened the perception of legitimate defensive action.

Another central requirement of Just War theory is last resort, which holds that all reasonable diplomatic options must be exhausted before military force is used. The American attack on Iran has been criticised on the grounds that diplomatic avenues, including negotiation and sanctions, had not completely failed. Critics therefore argue that the resort to military force may have been premature. A similar critique was levelled against the Iraq War, where inspections by the United Nations were still ongoing when the invasion began. By contrast, proponents of Israel’s actions in 1967 argue that the rapid deterioration of the strategic situation and the mobilisation of surrounding armies created conditions in which waiting could have resulted in catastrophic military disadvantage. Thus the Six-Day War is often presented as a case where leaders believed no viable diplomatic alternatives remained.

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Just War theory also requires proportionality and probability of success. The anticipated benefits of war must outweigh the foreseeable harm, and military action must have a reasonable chance of achieving its stated objectives. The Iraq War illustrates the dangers of misjudging these criteria. While the invasion rapidly toppled the Iraqi regime, the subsequent insurgency and regional instability produced costs far exceeding the initial expectations of policymakers. Critics fear that a similar dynamic could emerge in a prolonged conflict with Iran, given the country’s size, population, and capacity to mobilise regional proxy forces. By contrast, the Six-Day War achieved its immediate military objectives in a brief and decisive campaign. Although the long-term political consequences of the conflict remain deeply contested, the short-term military success arguably satisfied the criterion of probability of success more clearly than either Iraq or the current confrontation with Iran.

Finally, jus in bello governs the ethical conduct of military operations once war has begun. The principles of discrimination and proportionality require combatants to distinguish between military targets and civilians and to use force only to the extent necessary to achieve legitimate military objectives. All three conflicts have generated debate on this issue. Civilian casualties and infrastructural damage in Iraq drew sustained criticism from international observers, while contemporary reports of strikes in Iran have similarly raised concerns about the protection of civilian populations. Even in the Six-Day War, despite its rapid conclusion, allegations of excessive force and treatment of prisoners remain subjects of historical discussion. These debates demonstrate that even when the decision to enter war may appear justified, the manner in which the war is fought continues to raise profound ethical questions.

In conclusion the comparison of the United States attack on Iran with the Iraq War and the Six-Day War demonstrates both the continuing relevance and the growing difficulty of applying Just War theory in modern strategic environments. The classical framework developed by Augustine of Hippo and later refined by Thomas Aquinas assumed a world in which acts of aggression were relatively visible and where the boundaries between war and peace were more clearly defined. Contemporary conflicts, by contrast, frequently arise from ambiguous threats, intelligence assessments, and preventive strategic calculations. As a result, the criteria of just cause, last resort, and proportionality often become matters of interpretation rather than objective judgement.

“The house was shaking” – Iranians describe US attack. BBC

In the case of the U.S. action against Iran, the central ethical question concerns whether preventive or anticipatory force can satisfy the requirement of just cause. Classical Just War theory traditionally permitted war in response to aggression or an imminent threat but was deeply sceptical of wars initiated to eliminate possible future dangers. The debate surrounding Iraq in 2003 illustrates the consequences of stretching this principle. When the anticipated weapons of mass destruction failed to materialise, the moral and legal justification for the war was widely questioned, and the credibility of preventive war arguments was significantly weakened. If the American action against Iran rests on a similar preventive rationale, it risks reproducing the same moral controversy that characterised the Iraq conflict.

By contrast, the Six-Day War often appears in Just War discussions as a more plausible example of anticipatory self-defence. Israeli leaders believed that surrounding Arab forces were preparing for immediate hostilities, and that delaying action would place Israel at a severe strategic disadvantage. Whether that perception was entirely accurate remains debated among historians, yet the crisis atmosphere of May–June 1967 produced a situation in which the distinction between defensive and pre-emptive war became blurred but still recognisable. This contrast highlights an important lesson: Just War reasoning is heavily dependent upon the perceived immediacy and certainty of threat. Where the threat is clear and immediate, anticipatory action may be morally defensible. Where the threat is speculative or distant, the justification weakens considerably.

Iran attack on Israel AP News

Another important implication concerns legitimate authority and international order. Modern wars rarely occur in a legal vacuum; they take place within an international system shaped by institutions such as the United Nations and by evolving norms governing the use of force. The Iraq War demonstrated how the absence of broad international legitimacy can undermine even a militarily successful campaign. Similar questions arise in the case of Iran. Even if military action achieves limited tactical goals, the absence of widely recognised authority or consensus may erode the perceived legitimacy of the operation and complicate long-term strategic outcomes. Just War theory therefore remains relevant not only as a moral framework but also as a practical guide for sustaining international legitimacy.

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Finally, the comparison underscores the enduring importance of prudence, a virtue frequently emphasised within the Just War tradition. War may be morally permissible under certain conditions, but it remains an instrument of last resort whose consequences are inherently unpredictable. Iraq illustrates how initial military success can lead to prolonged instability and unintended geopolitical consequences. The Six-Day War demonstrates that even a rapid and decisive victory can generate complex and enduring political disputes. Any military confrontation with Iran therefore carries risks extending far beyond the immediate battlefield, including regional escalation, economic disruption, and long-term strategic rivalry.

For these reasons, the application of Just War theory to the American attack on Iran does not produce a simple or definitive verdict. Instead, it exposes the tensions between preventive security strategies and the moral restraints traditionally imposed on warfare. The framework continues to serve an essential purpose: it compels policymakers, military leaders, and scholars to examine not only whether war can be won, but whether it ought to be fought. In an era of increasingly complex geopolitical threats, this ethical discipline remains indispensable for evaluating the legitimacy and consequences of armed conflict.

Selected Bibliography

Michael Walzer, Just and Unjust Wars: A Moral Argument with Historical Illustrations (New York: Basic Books, 2015).

James Turner Johnson, Just War Tradition and the Restraint of War (Princeton: Princeton University Press, 1981).

Jeff McMahan, Killing in War (Oxford: Oxford University Press, 2009).

John Keegan, The Iraq War (London: Hutchinson, 2004).

Martin van Creveld, The Sword and the Olive: A Critical History of the Israeli Defense Force (New York: PublicAffairs, 1998).