The significant event that led to the founding of the United Nations was the end of World War II in 1945. In the wake of the war's devastation and the failures of the League of Nations to prevent such large-scale conflict, world leaders recognized the need for a new international organization to promote peace, security, and cooperation. The UN was established on October 24, 1945, with aims to foster diplomatic dialogue, address humanitarian issues, support international law, and prevent future conflicts. It has since become a crucial platform for addressing global challenges and promoting collective action across nations.