The question posed is the most important issue in world politics: under what circumstances may states lawfully resort to the unilateral (and international) use of force?
If Articles 2(4) and 51 of the Charter of the United Nations were in fact part of the living law of the world community, the question would be easy to answer. Then one could say with assurance that customary international law, jus gentium, the law distilled from the consent of the nations, had embraced Article 2(4) as a norm and allowed states to use force unilaterally only in exercising their inherent and sovereign rights of self-defense, which Article 51 says are not impaired in any way by the Charter, and in carrying out "decisions" of the Security Council. Under such circumstances, states could, for example, lawfully protect their citizens abroad, intervene in situations of chaos or near chaos to restore order, and participate in programs of collective self-defense like those of NATO or SEATO and the campaign to protect South Korea which began in 1950. Correspondingly, it could be said with confidence that states could not use force internationally to attack the territorial integrity or political independence of other states, nor allow their territory to be used for the purpose by armed bands, terrorists, or guerrillas, whether idealists or mercenaries, whose goal was to weaken or overthrow the government of the victim state.
Eugene V. Rostow,
The Legality of the International Use of Force by and from States,
Yale J. Int'l L.
Available at: http://digitalcommons.law.yale.edu/yjil/vol10/iss2/6