Worldwar: In the Balance

Harry Turtledove

World War II seethes across the planet. Hostilities spread in ever-widening circles of destruction: Nazi Germany Soviet Russia Britain France Japan Italy Africa - the fate of the world hung in the balance.
Then the real enemy arrives. Out of the skies comes an invasion force the like of which Earth has never known - and worldwar is truly joined. The invaders are inhuman and they are unstoppable. Their weaponry is overwhelming and their goal is simple: Fleetlord Atvar has arrived to claim Earth for the Empire.

'The wizard of If.' Chicago Sun-Times
'The standard-bearer for alternate history.' USA Today