What is the legality of U.S. strikes on Iran? | Explained

The story so far: On June 22, U.S. President Donald Trump launched military strikes on Iran, joining its ally Israel in efforts to derail Iran’s nuclear programme, which both countries claim is approaching weapons production. Iran retaliated the following day with missile attacks on Al-Udeid Air Base in Qatar, the forward headquarters of U.S. Central Command. After nearly two weeks of escalating hostilities, Iran and Israel agreed to a ceasefire on June 24.

What is a lawful exercise of self-defence?

The UN Charter, under Article 2(4), prohibits the use of force against the territorial integrity or political independence of any state, except in narrowly defined circumstances — a claim of self-defence under Article 51 or with the UN Security Council’s (UNSC) authorisation. The restrictive interpretation, grounded in the text of Article 51, permits self-defence only in response to an armed attack that is already under way. A more permissive interpretation allows for self-defence in response to an armed attack that is imminent. This broader interpretation, often referred to as anticipatory self-defence, has been endorsed in several UN-affiliated reports. Notably, the 2004 report of the Secretary-General’s High-Level Panel on Threats, Challenges and Change affirmed that “a threatened State, according to long-established international law, can take military action as long as the threatened attack is imminent, no other means would deflect it, and the action is proportionate”. These criteria are derived from the famous Caroline case, which established that the use of force is lawful only when the need for self-defence is “instant, overwhelming, leaving no choice of means, and no moment for deliberation”. Over time, many states have argued that the Caroline standard is too rigid to address contemporary security threats.

This has led to attempts to reinterpret and expand the notion of imminence, giving rise to the controversial doctrine of pre-emptive self-defence. Under this doctrine, a state may use force not only in response to an attack that is imminent but also during what is perceived as the “last window of opportunity” to neutralise a threat posed by an adversary with both the intent and capability to strike. The U.S. has been a leading proponent of this doctrine, invoking it to justify the 2003 invasion of Iraq. “Pre-emptive self-defence lacks the requisite state practice and opinio juris to qualify as customary international law. States are generally reluctant to endorse its legality, as the absence of an imminent threat renders the doctrine highly susceptible to misuse,” Prabhash Ranjan, Professor at Jindal Global Law School, told The Hindu.

Did Iran pose an ‘imminent’ threat?

The U.S. has not submitted an Article 51 notification to the UNSC declaring its strikes on Iran as self-defence. However, U.S. Defense Secretary Pete Hegseth described them as a precision operation to neutralise “threats to national interest” and an act of “collective self-defence” of U.S. forces and its ally, Israel.

Tehran has maintained that its nuclear programme is for civilian purposes and remains under the supervision of the International Atomic Energy Agency. However, on June 12, the UN nuclear watchdog passed a resolution accusing Iran of violating its non-proliferation obligations, while noting that inspectors have been unable to confirm whether the programme is “exclusively peaceful”.

In March, Director of National Intelligence Tulsi Gabbard initially told Congress that while Iran had stockpiled materials, it was not actively building a nuclear weapon. However, she later warned that Iran could do so “within weeks,” after President Trump claimed Iran could develop one “within months.”

Dr. Ranjan noted that the criteria for determining an ‘imminent threat’ remain highly contested, as the International Court of Justice (ICJ) has never ruled on the legality of anticipatory self-defence or pre-emptive strikes. “For the U.S. to credibly invoke pre-emptive self-defence, it must present clear evidence of both Iran’s intent and capability to strike in the near future. This is a difficult threshold to meet, given that Iran does not yet possess a nuclear weapon,” he said. He added that ongoing U.S.-Iran negotiations indicate that diplomatic means were still available.

What about collective self-defence?

Under Article 51 of the Charter, Israel can call on the assistance of its allies to exercise collective self-defence against an attack. “Israel’s strikes on Iran, framed as pre-emptive action against perceived nuclear threats, are legally suspect. This, in turn, casts doubt on the legitimacy of any claim to collective self-defence,” Dr. Ranjan said. Israel has also sought to justify its military offensive as part of an “ongoing armed conflict,” citing a history of attacks by groups like Hamas and the Houthis, which it claims act as Iranian proxies. However, to legally sustain this argument, Israel must meet the “effective control” test set by the ICJ in Nicaragua versus U.S. (1986). This is a high threshold to meet since it requires proof that Iran exercises “overall control” over these groups beyond merely funding or arming them.

What are the implications?

Allowing states to invoke pre-emptive self-defence would effectively grant powerful nations the licence to unilaterally use force based on mere conjecture. This would further weaken the already fragile rules-based international order. It is, therefore, crucial to resist expanding legal definitions of what constitutes an imminent threat, particularly when punitive action by the UNSC against permanent members like the U.S. remains unlikely due to their veto power.