When did World War II begin?

Adolf Hitler had expanded German territory through the late 1930s, annexing Austria and taking over Czechoslovakia while Britain and France pursued appeasement. After securing a non-aggression pact with the Soviet Union in August 1939, Germany attacked Poland from the west on 1 September. Britain and France, which had guaranteed Polish independence, declared war on 3 September 1939, and the Soviet Union invaded Poland from the east on 17 September. The war widened dramatically in 1941 with the German invasion of the Soviet Union in June and the Japanese attack on Pearl Harbor in December, which brought the United States in. It became the deadliest conflict in history, with an estimated 70 to 85 million deaths.