Accordingly, our respective Governments, through representatives assembled in the city of San Francisco, who have exhibited their full powers found to be in good and due form, have agreed to the present Charter of the United Nations and do hereby establish an international organization to be known as the United Nations.
- UN Charter
The United Nations (UN) is a multinational organization, with nearly every state in the world as a voting member. It was started at the United Nations Conference on International Organization in 1945 in San Francisco. The charter was opened for signature on June 26, 1945, and ratified by the members of the Security Council on October 24, 1945, at which point it entered into force.
Wikipedia seems to suggest like three different locations for local buildings where this happened, so someone please research.