Self-defence in international law

Article 2(4) of the UN Charter

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.

— Chapter I, Article 2(4) of the UN Charter

The drafters’ intent was that collective force approved and organized by the Security Council would substitute for unilateral uses of force by states. However, some states were concerned that use of the veto power by one of the Council's permanent members might prevent that body from taking necessary action, and they insisted upon inserting into the Charter an explicit right of self defence.

Customary international law and Caroline test

The traditional customary rules on preemptive self-defence derive from an early diplomatic incident between the United States and the United Kingdom over the killing of a US citizen (a Black American watchmaker named Amos Durfee) who was on board a ship (the Caroline), which was docked in the U.S. but which had been carrying personnel and stores of war to rebels in Canada, then a British colony. The U.S. government had not approved or supported the Caroline's activities and the ship was peacefully at anchor in the U.S. when British forces attacked, burned the ship and sent it over Niagara Falls. The so-called Caroline case established that in order for one state to use force in the territory of another state which had not used force first there had to exist "a necessity of self-defence, instant, overwhelming, leaving no choice of means, and no moment of deliberation,' and furthermore that any action taken must be proportional, "since the act justified by the necessity of self-defence, must be limited by that necessity, and kept clearly within it." These statements by the US Secretary of State to the British authorities are accepted as an accurate description of the customary right of preemptive, or anticipatory, self-defence. They are sometimes mistakenly said to apply to all uses of force by states in self-defence.

Imminent threat

Imminent threat is a standard criterion in international law, developed by Daniel Webster as he litigated the Caroline affair. An imminent threat is "instant, overwhelming, and leaving no choice of means, and no moment for deliberation". This criterion is used in the international law justification of preemptive self-defence: self-defence without being physically attacked first (see Caroline test). This concept has been used to mitigate the lack of definition provided by Article 51 of the Charter of the United Nations, which states that sovereign nations may fend off an armed attack until the Security Council has adopted measures under Chapter VII of the United Nations Charter.

The Caroline affair has been used to establish the principle of "anticipatory self-defence" and is also now invoked frequently in the course of the dispute around preemptive strike (or preemption doctrine).

Historical

Hugo Grotius, the 17th-century jurist and father of public international law, stated in his 1625 magnum opus The Law of War and Peace that "Most Men assign three Just Causes of War, Defence, the Recovery of what's our own, and Punishment."