Hitler's War

The master of alternative history asks the question, 'What would have happened if World War II had started in 1938?'. The results are thrilling.
The two sides of the Spanish civil war are still locked in a blood-soaked stalemate. Stalin's purge of the Red Army is barely underway. And Neville Chamberlain - sickened by the arrogance and duplicity of the Germans- does not return from Munich waving the piece of paper that would give the Czech arms factories to Hitler and postpone the war until 1939.
On October 1, German tanks cross the Czech frontier, touching off declarations of war from France, from England, from the USSR. Poland, fearing the Russians more than Hitler, declares war on the German side. Soon Fascist Spain attacks Gibraltar, the Japanese army crosses the Manchurian frontier into Siberia . . . and the British Army sets off for France, which has launched a pre-emptive attack on the Rhineland.
The war we know as World War II has begun - a year early, in an entirely different way.

Harry Turtledove