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.