The major global conflict that began in 1939 and ended in 1945 is World War II. It involved most of the world’s nations, split into the Allies, including the United States, the Soviet Union, and the United Kingdom, against the Axis powers, primarily Germany, Italy, and Japan. The war was marked by unprecedented levels of violence, including the Holocaust and the use of atomic bombs. It fundamentally reshaped international relations, led to the establishment of the United Nations, and initiated the Cold War. World War II's consequences continue to influence global politics and history today.