Following ratification by the five permanent Security Council members and a majority of other signatories, the United Nations Charter entered into force, creating an international body meant to prevent another world war. Fifty-one nations became founding members, meeting first in San Francisco earlier that year to hammer out the document. October 24 has been celebrated as United Nations Day ever since.
The horrors of two world wars in a single generation convinced diplomats that the League of Nations' failure could not be repeated. Delegates from fifty nations spent two months in San Francisco that summer drafting a charter meant to bind great powers into permanent cooperation, giving the US, UK, France, China, and the Soviet Union veto power on the Security Council as the price of their participation.
When the charter finally cleared ratification by the required majority, including all five permanent members, it took legal effect on October 24, 1945. Secretary-General positions and headquarters logistics were still unsettled; the organization would not occupy its Manhattan headquarters for another six years. Yet the date stuck as the UN's official birthday, marked annually with ceremonies from Geneva to New Delhi.
The institution that emerged has been alternately hailed as humanity's best hope for collective security and derided as a toothless talking shop, unable to stop wars from Rwanda to Syria. Both criticisms miss that its founders never intended it to override sovereign power, only to create a forum where catastrophe might be averted through negotiation rather than another global bloodletting.