If the first war got away from the men who started it, the second is aimed on purpose. It is several wars at once: a German war of conquest and extermination, a Japanese bid for an Asian empire, a Chinese struggle already years old, and a colonial world asked to fight for freedoms it does not have.
Why then
The first war's peace left humiliation, unpaid debts and borders nobody accepted. The Depression made men who promised to tear it all up look like answers. In Europe the war begins in Poland in 1939; in Asia it had begun in 1937, or in 1931.
What they wanted
Hitler wants a racial empire in the east. Tokyo is after a resource sphere it calls co-prosperity. Churchill, Roosevelt, Stalin and Chiang fight first to survive, and then for postwar maps that do not match.
What changed
Seventy to eighty-five million people are killed, most of them civilians. Six million Jews are murdered in the Holocaust. The winners end it with the bomb, the United Nations and occupation zones; Europe's empires are too weak to hold Asia; America and the Soviet Union face each other across a divided Germany.
Still argued
One war, or several joined in 1941? Made inevitable at Versailles, or chosen in Berlin and Tokyo in the 1930s? Historians who start the story in China tell a different war from those who start in Poland.