Worldwar, book 1 : In the balanceHarry Turtledove1993

From Pearl Harbor to panzers rolling through Paris to the Siege of Leningrad and the Battle of Midway, war seethed across the planet as the flames of destruction rose higher and hotter.
And then, suddenly, the real enemy came.
The invaders seemed unstoppable, their technology far beyond human reach. And never before had men been more divided. For Jew to unite with Nazi, American with Japanese, and Russian with German was unthinkable.
But the alternative was even worse.
As the fate of the world hung in the balance, slowly, painfully, humankind took up the shocking challenge . . .