The standing organization of nearly all the world's states, founded in 1945 from the wreckage of the Second World War to keep peace and coordinate cooperation. It pairs a universal General Assembly, where every state has one vote, with a powerful Security Council whose five permanent members can each veto any action. That built-in compromise is what lets the organization function and what guarantees it will fail whenever a great power blocks it.
In 1945, the men who designed the United Nations did something that looks, at first, like a betrayal of their own ideals. They wrote a Charter promising that all nations were equal, and then they handed five of them a button that could stop the whole machine. That button is the veto, and the five who hold it were the victors of the Second World War. The designers were not being cynical. They had watched the League of Nations collapse because the great powers would not stay inside it. So they built the great powers a throne and dared them to remain seated. Everything the United Nations can do, and everything it has failed to do, flows from that one decision.