World War II was a global conflict lasting from 1939 to 1945, involving most of the world's nations. This includes all of the great powers, which eventually formed two opposing alliances, the Allies and the Axis. It was the most widespread war in history, with more than 100 million soldiers sent out. This was the deadliest conflict in human history, with several tragic events including the Holocaust and the only time nuclear weapons have been used in warfare.
The war is thought so have begun on September 1, 1939, with the invasion of Poland by Germany. From that day forward, Germany set out to gain a large empire in Europe. From late 1939 to early 1941, in a series of treaties, Germany conquered much of Europe.