The legality of the use of force remains one of the most debated issues in contemporary international law. The relationship between the United States of America and the Islamic Republic of Iran has repeatedly raised difficult legal questions concerning the interpretation of the United Nations Charter, particularly Articles 2(4), 39, 42 and 51. Since the Iranian Revolution of 1979, both states have accused each other of unlawful intervention, threats to sovereignty, support for proxy warfare, and violations of international peace and security. The legal controversies intensified after incidents such as the Tanker War in the Persian Gulf, the shooting down of Iran Air Flight 655, cyber operations against Iranian nuclear facilities, attacks on Gulf shipping, the assassination of General Qassem Soleimani in January 2020, and Iranian retaliatory strikes on United States military installations in Iraq.[UN Charter, Art 2(4).]

This research paper critically examines the legality of the use of force in the context of the USA–Iran conflict under the framework of the United Nations Charter and customary international law. It analyses the prohibition on the use of force under Article 2(4), the doctrine of self-defence under Article 51, the role of the United Nations Security Council, and the jurisprudence of the International Court of Justice. The paper evaluates competing legal arguments advanced by the United States and Iran and examines whether the conduct of either state can be justified under existing principles of international law.[Ian Brownlie, International Law and the Use of Force by States (Oxford University Press 1963) 112.][Christine Gray, International Law and the Use of Force (4th edn, Oxford University Press 2018) 30.]

The paper argues that the USA–Iran conflict illustrates both the strength and the fragility of the collective security system established by the UN Charter. Although the Charter provides a comprehensive prohibition against unilateral force, state practice has increasingly relied on expanded interpretations of self-defence, anticipatory self-defence, and the protection of national security interests. The paper concludes that many actions undertaken by both the United States and Iran raise serious concerns regarding compatibility with the UN Charter.[Yoram Dinstein, War, Aggression and Self-Defence (7th edn, Cambridge University Press 2021) 87.]