LAURENCE REES: Why did the war happen?

SIR IAN KERSHAW: Well, when you say the war you mean the Second World War, and it was a world war. It depends on your perspective, but it became a world war with the entry of the United States and Japan into this war in December 1941. But the war had been running in Europe since September 1939, and it had been running in the Far East since the beginning of the Japanese-Chinese war in July 1937. So you have two wars at opposite ends of the globe which then become a world war in December 1941. But the short answer to the question, therefore, is that this war happened because of the expansionist aims and policies of Germany and of Japan.