The document that established the United Nations in 1945 is the United Nations Charter. This foundational treaty was signed by 51 nations on June 26, 1945, in San Francisco, and came into force on October 24, 1945. The Charter outlines the purposes, principles, and structure of the UN, aiming to promote international cooperation, peace, and security following the devastation of World War II. It emphasizes the importance of respecting human rights and the self-determination of nations. The establishment of the UN marked a significant step toward global governance and a commitment to preventing future conflicts through diplomatic means.