The key factor that led to the establishment of the United Nations in 1945 was the failure of the League of Nations and the devastation caused by World War II. The international community recognized the need for a more robust and effective organization to promote peace, security, and cooperation among nations. The aim was to prevent future conflicts and provide a platform for dialogue. The UN was established to encourage diplomacy, provide humanitarian aid, and uphold international law, making it a pivotal component of global governance after the war.