The significant event that led to the establishment of the United Nations was World War II, which catalyzed a global consensus on the need for international cooperation to prevent future conflicts. The San Francisco Conference, held in April-June 1945, brought together representatives of 50 nations to draft the UN Charter. This Charter was signed on June 26, 1945, and officially came into effect on October 24, 1945. The UN was formed to promote peace, security, and cooperation among nations, reflecting lessons learned from the devastating war.