Israel’s Defence Minister Israel Katz said his country may soon have to “act again” against Iran, to ensure the Islamic republic “does not once again become a threat to Israel”. “U.S. President Donald Trump, in coordination with (Israeli) Prime Minister Benjamin Netanyahu, is leading the efforts to achieve the campaign’s objectives, to ensure that Iran does not once again become a threat to Israel, the United States and the free world in the future,” media reports cited him saying on Thursday (April 30) during a military ceremony.Does Israel’s reasoning justify the US-Israel war against the Islamic Republic? What does the just war theory say?Also Read | Beyond Trending: What is strategic ambiguity?Just war theoryJust war theory is an ethical framework that concerns itself with why and how wars are fought. The theory is traced to medieval Christian just war tradition, where St. Augustine and Thomas Aquinas made significant contributions to its development. While Augustine’s comments offer a biblical rationale for war, Aquinas delineates the general principles of the just war theory. Aquinas engages with the question of not only the justification of war but also with the conduct during the war. These two questions constitute the two key components of the theory, referred to by their Latin names: Jus ad bellum – Outlining the justifications for entering a war.Jus in bello – Governing the conduct during warfare. The theory evolved over the years to incorporate both theological and secular perspectives on warfare, but the framework suggested by Aquinas continues to define it. It largely revolves around key principles of jus ad bellum and Jus in bello that distinguish just wars from unjust ones.Story continues below this adKey principles of jus ad bellumKey principles of jus in belloJust cause Legitimate authorityRight intentionReasonable prospects for successProportionality Last resortDistinction between targets and othersProportionalityNecessityThus, for a war to be defined as just, it must be declared by a legitimate authority for a just cause (self-defence) with right intention (for peace) and a fair chance of achieving the goal. The war must be declared as a last resort, and the harm caused by it must not be greater than the good sought. Must Read | Beyond Trending: What is war of attrition?Jus in bello governs the conduct during the war. It requires that civilians must be protected, and force must be used when necessary without causing disproportionate harm. A number of authors have also introduced a third component in the just war theory – jus post bellum or justice after war.Revisionists critique Although the just war tradition is seen as a valuable tool for thinking about the morality of war, one of the most dominant traditions in international relations – realism – prioritises realpolitik and the pursuit of power over morality. Story continues below this adTherefore, such moral scepticism is seen as presenting a direct and immediate challenge to just war thinking, argues David Fisher in his book Morality and War: Can War be Just in the Twenty-first Century?. At the same time, revisionists, like Jeff McMahan and David Rodin, have raised several questions, including the moral equality of combatants and their moral duties to one another. For instance, soldiers fighting on either side of the war are treated as legitimate targets by both traditionalists and international law and therefore are entitled to moral protection. But revisionists argue that soldiers fighting for an unjust cause and posing unjust threats are not entitled to moral protections. In addition, the changing nature of warfare and the emergence of weapons of mass destruction complicate the principles like proportionality, distinction, just cause, and reasonable prospects of success of the just war theory. For instance, the NATO’s campaign in Kosovo, the US invasion of Iraq, the Israel-Palestine war and the ongoing US-Israel war against Iran invite intense debate about the principles of just war theory. Nonetheless, the theory provides a robust and indispensable guide for addressing the security challenges of the twenty-first century, says Fisher. Story continues below this adWe would love to hear what you think about this new initiative. Send your comments at ashiya.parveen@indianexpress.com.Click Here to read the UPSC Essentials magazine for April 2026. Subscribe to our UPSC newsletter and stay updated with the news cues from the past week.Stay updated with the latest UPSC articles by joining our Telegram channel – IndianExpress UPSC Hub, and follow us on Instagram and X.