The significant event that led to the formation of the United Nations (UN) was the conclusion of World War II in 1945. In the aftermath of the war, it became evident that existing international mechanisms, like the League of Nations, had failed to prevent the atrocities and global conflict. The UN was established to promote peace, security, and cooperation among nations while addressing issues such as human rights, humanitarian aid, and sustainable development. The founding charter was signed on June 26, 1945, in San Francisco, bringing together diverse nations in a collective effort to prevent future conflicts and foster international collaboration.