The thesis discusses whether a violation of international law is a necessary element of a threat to the peace under Art. 39 of the UN Charter. This question is of great practical importance, particularly in relation to international terrorism and armament with weapons of mass destruction. The author begins with an analysis of the resolutions of the Security Council and an interpretation of the UN Charter. Having come to the conclusion that a violation of international law is a prerequisite for enforcement measures, the author then examines the requirements regarding the violation of international law, effects on the effectiveness of the measures of the Security Council and potential exceptions.