The high ambition of the UN Charter Art. 2:4 to prevent interstate use of force is confirmed by the narrow right to individual or collective self-defence given to states under Art.51 ‘if an armed attack occurs’. States have many times invoked ‘self-defence’ as default justification of interventions and sometimes sought room for interpretations giving them more elbow room to use force in response to hostile acts of gravity. The general right of reprisal has been rejected, but in cases of grave or repeated terrorist attacks responses by armed force have not been criticized even though taken much after the attacks. A right has also been recognized to use force in self-defence where armed attacks are ‘imminent’ (pre-emption), but no such right has been acknowledged where the threat of attack is not imminent (‘anticipatory self-defence’ or ‘preventive’ use of force).

The interstate use of force aiming at the acquisition of territory has been shown to have been rare after WWII. Interventions – actions without that aim but relying on open or shrouded or subversive use force – have remained common. Are they all illegal under Art. 2:4? Concrete cases are examined of interventions undertaken in circumstances not foreseen at the adoption of the Charter. They show an unwillingness in the UN to find force aimed at decolonization illegal. Contrived justifications for interventions undertaken in pursuit of the Cold War rejected, while understanding is shown for interventions using force in response to terrorism. Attacks – including interventions – that are imminent are deemed to justify use of force in self-defence, but attacks not seen as imminent do not. In line with this thinking, the use of force to destroy a nuclear research reactor that could have produced plutonium for future nuclear weapons was condemned by the Security Council in 1981. Attacks made with similar intentions – by cyber means and by assassinations – have nevertheless been undertaken.