Article 51 of the UN Charter

The right of self-defense in international law is recognized by the United Nations Charter, specifically in Article 51 of Chapter VII, which allows states to use force in self-defense against an armed attack. The customary international law of self-defense, established in the Caroline case, requires a necessity of self-defense that is instant, overwhelming, and leaves no choice of means, and any action taken must be proportional and limited by that necessity. The concept of imminent threat, developed in the Caroline affair, is a standard criterion for international law justification of preemptive self-defense, which allows states to fend off an armed attack without being physically attacked first.learn more on wikipedia