Harry Turtledove 
Worldwar: In the Balance 

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