When the UN Security Council voted Wednesday, March 11<\/a>, condemning Iran\u2019s missile and drone strikes across the Gulf, the resolution hinged on Chapter VII, Article 51 of the UN Charter.<\/a><\/p>\n The text of Resolution 2817<\/a> condemns Iran\u2019s \u201cegregious attacks\u201d on Bahrain, Kuwait, Oman, Qatar, Saudi Arabia, the United Arab Emirates and Jordan, and reaffirms those states\u2019 \u201cinherent right of individual or collective self-defense.\u201d<\/p>\n As the U.S.-Israel war against Iran spreads across the Gulf, Article 51 is becoming the central legal framework through which governments are interpreting the conflict.<\/p>\n It\u2019s also being tested and invoked more frequently than at any time in decades.<\/p>\n Restraining Aggression<\/p>\n Article 2(4)<\/a> of the UN Charter requires states to refrain from \u201cthe threat or use of force against the territorial integrity or political independence of any state\u201d with two explicit exceptions.<\/p>\n The first is when Member States of the Security Council authorize force under Chapter VII<\/a>.<\/p>\n The second is when one or more Member States claim self-defense under Article 51, which states:<\/p>\n \u201cNothing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations\u2026\u201d<\/p>\n An Increasingly Common Claim<\/p>\n In recent years, governments have invoked Article 51 far more frequently than in the past.<\/p>\n In fact, in an official communication<\/a>\u00a0to the Security Council, Mexico\u2019s diplomatic mission noted that the provision has been cited at least 78 times<\/a> since 2021 \u2013 a sharp increase compared with earlier decades.<\/p>\n Several recent examples illustrate how broadly the doctrine is being used.<\/p>\n In June 2025, the U.S. launched strikes on Iranian nuclear facilities during Operation Midnight Hammer<\/a>. In a formal Article 51 letter<\/a> to the Security Council, Washington argued the attacks were \u201cnecessary and proportionate\u201d measures to defend Israel and U.S. security after diplomatic efforts failed.<\/p>\n Days earlier, Israel carried out its own strikes inside Iran. A communiqu\u00e9<\/a> from the Israeli Foreign Minister described the operation as necessary to thwart \u201cimminent Iranian attacks,\u201d language understood as invoking anticipatory self-defense, though they did expressly mention Article 51.<\/p>\n In September, Israel also carried out a strike<\/a> targeting Hamas leadership in Qatar, framing the action as part of its continuing self-defense following the October 7, 2023 attacks.<\/a><\/p>\n Even operations far from traditional battlefields have been justified in this way. A U.S. strike against a vessel allegedly involved in drug trafficking in the Caribbean was defended in a war powers report<\/a> as self-defense against threats emanating from states \u201cunable or unwilling\u201d to address them.<\/p>\n Necessity and Proportionality<\/p>\n Two conditions must be met for a use of force to be permissible under Article 51:<\/p>\n Necessity \u2014 force must be required to repel or prevent an armed attack.<\/p>\n Proportionality \u2014 the response must be limited to what is necessary to address the threat.<\/p>\n These principles were reinforced by the International Court of Justice in cases such as Nicaragua v. United States<\/a> and Oil Platforms (Iran v. United States).<\/a> Together, they serve as the legal yardstick for determining whether military action qualifies as legitimate self-defense or unlawful aggression.<\/p>\n The Debate Over \u201cImminent\u201d Attacks<\/p>\n Disagreements frequently arise over when the right of self-defense actually begins.<\/p>\n Many countries argue the Charter is clear \u2013 self-defense applies only after an armed attack has occurred.<\/p>\n That interpretation is widely supported by countries in the Global South and was reaffirmed at the 2024 summit of the Non-Aligned Movement<\/a>. (Founded in 1961, NAM is a forum of 120 nations not formally aligned with or against any major global power).<\/p>\n The U.S. and several allies take a broader view, arguing that states can act when an attack is imminent \u2013 allowing military force to prevent a strike before it happens.<\/p>\n The \u201cUnwilling or Unable\u201d Doctrine<\/p>\n The most controversial extension of Article 51 involves attacks on non-state actors operating inside another country\u2019s territory.\u00a0Under this \u201cunwilling or unable\u201d doctrine, a state may use force in another country if that government cannot or will not prevent attacks originating there.<\/p>\n