In 1945, the world was unanimously tired of fighting. The general hope by establishing the United Nations was that future generations could be spared the horrors of war. After all, millions of people, including hundreds of thousands of Americans, had laid down their lives in the years prior. It was Roosevelt's belief that peace could be maintained by the allied powers of the war, the United States, the Soviet Union, China, France, and Great Britain.

The U.N. established something called the Security Council, which was granted the power to take action to maintain peace. Each of the previously mentioned five allied countries had a veto power. All it took was a "no" from one of these countries and the Security Council could not act. This, of course, was a problem in the wake of the Cold War. Communist and non-communist countries were constantly fighting, and since both the United States and the Soviet Union had vetoing power, the Security Council could never act.

Because of the Cold War, the U.N. remained at a standstill during some very important and threatening events in world history. During the Cuban Missile Crisis, for example, the Security Council did nothing.

Before the Cold War, the United Nations was supposed to be a body that would be able to maintain peace in the world.  It was supposed to represent the consensus of all nations.  At the end of WWII, there were no formal rivalries between any major countries and so it was hoped that the UN would represent all countries in the world.

As the Cold War got started, this fell apart.  The world started to split up into the Communist bloc and the US and its allies.  This led to the UN being unable to do anything major because the two sides would never agree.  So instead of being a major peacekeeping organization run on consensus, the UN had to be content with doing minor, noncontroversial things like UNICEF.