There was little collective security action taken by the UN after WWII.What was the fundamental or most significant cause of this phenomenon?

Question

Quizplus · Accepted Answer

The veto power of the permanent members of the Security Council (United States, United Kingdom, France, China, and later the Soviet Union/Russia) was often used to block collective security action, as these members were reluctant to take actions that might be against their own interests. This led to a lack of effective action by the UN to prevent or resolve conflicts. The instability caused by the Cold War and the UN's aversion to military action were contributing factors, but the veto power of the Security Council members was the most significant cause. The break-up of the USSR occurred after the end of the Cold War and did not have a direct impact on the UN's collective security efforts.

There Was Little Collective Security Action Taken by the UN