World War II, or the Second World War, was the second war involving many participants, including the UK. It was the most widespread war in history, with more than 100 million people serving in military units. But what were the circumstances which brought it about? Germany was ruined by its defeat in World War I and this lead to the rise of Adolf Hitler and his Nazi Party. Hitler wanted to conquer the neighboring lands and this brought about World War II. Between 50 to 70 million people were killed in the war and it is the only conflict in which nuclear weapons have been used.