Israel’s major military operation against Iran has targeted its nuclear program, including its facilities and scientists, as well as its military leadership.In response, the United Nations Security Council has quickly convened an emergency sitting. There, the Israeli ambassador to the UN Danny Danon defended Israel’s actions as a “preventative strike” carried out with “precision, purpose, and the most advanced intelligence”. It aimed, he said, to: dismantle Iran’s nuclear programme, eliminate the architects of its terror and aggression and neutralise the regime’s ability to follow through on its repeated public promise to destroy the state of Israel.So, what does international law say about self-defence? And were Israel’s actions illegal under international law?When is self-defence allowed?Article 2.4 of the UN charter states:All members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations.There are only two exceptions:when the UN Security Council authorises force, andwhen a state acts in self-defence. This “inherent right of individual or collective self-defence”, as article 51 of the UN charter puts it, persists until the Security Council acts to restore international peace and security. So what’s ‘self-defence’ actually mean?The International Court of Justice (ICJ) has consistently interpreted self-defence narrowly.In many cases, it has rejected arguments from states such as the United States, Uganda and Israel that have sought to promote a more expansive interpretation of self-defence.The 9/11 attacks marked a turning point. The UN Security Council affirmed in resolutions 1368 and 1373 that the right to self-defence extends to defending against attacks by non-state actors, such as terrorist groups. The US, invoking this right, launched its military action in Afghanistan.The classic understanding of self-defence – that it’s justified when a state responds reactively to an actual, armed attack – was regarded as being too restrictive in the age of missiles, cyberattacks and terrorism.This helped give rise to the idea of using force before an imminent attack, in anticipatory self-defence.The threshold for anticipatory self-defence is widely seen by scholars as high. It requires what’s known as “imminence”. In other words, this is the “last possible window of opportunity” to act to stop an unavoidable attack. As set out by then-UN Secretary-General Kofi Annan in 2005:as long as the threatened attack is imminent, no other means would deflect it and the action is proportionate, this would meet the accepted interpretation of self defence under article 51.As international law expert Donald Rothwell points out, the legitimacy of anticipatory self-defence hinges on factual scrutiny and strict criteria, balancing urgency, legality and accountability.However, the lines quickly blurredIn 2002, the US introduced a “pre-emptive doctrine” in its national security strategy.This argued new threats – such as terrorism and weapons of mass destruction – justified using force to forestall attacks before they occurred.Critics, including Annan, warned that if the notion of preventive self-defence was widely accepted, it would undermine the prohibition on the use of force. It would basically allow states to act unilaterally on speculative intelligence. Annan acknowledged:if there are good arguments for preventive military action, with good evidence to support them, they should be put to the Security Council, which can authorise such action if it chooses to. If it does not so choose, there will be, by definition, time to pursue other strategies, including persuasion, negotiation, deterrence and containment – and to visit again the military option.This is exactly what Israel has failed to do before attacking Iran.Lessons from historyIsrael’s stated goal was to damage Iran’s nuclear program and prevent it from developing a nuclear weapon that could be used against it. This is explicitly about preventing an alleged, threatened, future attack by Iran with a nuclear weapon that, according to all publicly available information, Iran does not currently possess.This is not the first time Israel has advanced a broad interpretation of self-defence.In 1981, Israel bombed Iraq’s Osirak nuclear reactor, which was under construction on the outskirts of Baghdad. It claimed a nuclear-armed Iraq would pose an unacceptable threat. The UN Security Council condemned the attack. As international law stands, unless an armed attack is imminent and unavoidable, such strikes are likely to be considered unlawful uses of force.While there is still time and opportunity to use non-forcible means to prevent the threatened attack, there’s no necessity to act now in self defence.Diplomatic engagement, sanction, and international monitoring of Iran’s nuclear program — such as through the International Atomic Energy Agency — remain the lawful means of addressing the emerging threat posed by Tehran.Preserving the rule of lawThe right to self-defence is not a blank cheque.Anticipatory self-defence remains legally unsettled and highly contested. So were Israel’s attacks on Iran a legitimate use of “self-defence”? I would argue no.I concur with international law expert Marko Milanovic that Israel’s claim to be acting in preventive self-defence must be rejected on the facts available to us. In a volatile world, preserving these legal limits is essential to avoiding unchecked aggression and preserving the rule of law.Shannon Bosch does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.