The Second World War was a global conflict that lasted from 1939 to 1945, involving most of the world's nations. It was marked by significant events such as the Holocaust, the use of atomic bombs on Hiroshima and Nagasaki, and the formation of the United Nations after the war to promote peace and cooperation. The war was primarily fought between two major alliances: the Allies and the Axis Powers. It resulted in an estimated 70-85 million casualties, making it one of the deadliest conflicts in human history. The aftermath of the war reshaped international relations and led to the Cold War era.