World War II lasted from 1939 to 1945 and involved many major countries. Germany invaded Poland in 1939 starting the war in Europe. The US joined in 1941 after Japan's attack on Pearl Harbor. Major Allied forces included the US, UK, France and Soviet Union against the Axis powers of Germany, Italy and Japan. Over 60 million people died making it the deadliest conflict in history. Germany surrendered in 1945 after the US dropped atomic bombs on Japan, ending the war.