The United Nations is an international organization of all nations of Earth created for ending armed conflicts, world poverty, diseases, etc, and creating a new era of peace and prosperity on Earth.
The United Nations was created at the end of World War II, thereby effectively replacing the earlier League of Nations. Very much is known of the United Nations' history and contributions during the Cold War. Nearly all of the nations joined the UN for cooperation and prosperity.
Before the outbreak of World War III, the U.N. had done everything they can to keep the war from breaking out by trying to help the Soviet Union from collapsing by giving economic assistance which they didn't know they'd been blackmailed. But soon as diplomacy had failed, it would mean that the armies took over and prepared for war.
As the war broke out, the U.N. had tried everything they can to stop the fight, but it was too late. The war had destroyed all the efforts of peace as the US-NATO forces fought the Soviet Army everywhere on the globe. As other nations opted not to intervene, the other nations joined the war either on the side of the U.S. or the Soviet Union.
Much of their exploits during the war is currently unknown.