In what year did World War II start?

World War II officially started in Europe in 1939, following the Polish invasion by Adolf Hitler and prompting Britain and France to formally declare war against Germany. The United States joined the Allied forces in 1941.

World War II saw the rise to power of the Nazi Party under the dictatorial leadership of Adolf Hitler. On September 1, 1939, Hitler ordered the "blitzkrieg" attack on Poland and the summary execution of its people. Neither the British, nor the French government took decisive measures against the Germans, despite the proclamation of war. The participation of the U.S. in the second world war was triggered by the Japanese attack on Pearl Harbor, which took place on December 7, 1941.