The major global event that took place from 1939 to 1945 was World War II, a conflict involving most of the world's nations. It began with Germany's invasion of Poland and involved multiple theaters of war, including Europe, Africa, and the Pacific. The war was characterized by significant events such as the Holocaust, the use of atomic bombs on Hiroshima and Nagasaki, and the formation of alliances like the Axis and Allies. World War II fundamentally altered the political landscape, leading to the emergence of the United States and the Soviet Union as superpowers and setting the stage for the Cold War.