The major world event that began in 1939 and ended in 1945 is World War II. This global conflict involved the majority of the world's nations, including the great powers, divided into the Allies and the Axis. It was triggered by the aggressive expansionist policies of Nazi Germany, led by Adolf Hitler. The war saw significant events, such as the Holocaust, the use of atomic bombs, and major battles like Stalingrad and Normandy. It resulted in immense destruction and loss of life, fundamentally reshaping international relations and leading to the establishment of the United Nations and the Cold War.