Upsetting the Balance — Harry Turtledove

NO ONE COULD HALT THE ENEMY'S ADVANCE.
NOT CHINA OR THE SOVIET UNION,
NOT JAPAN OR NAZI GERMANY, NOT EVEN THE UNITED STATES.

From Warsaw to Moscow to China's enemy-occupied Forbidden City, the nations of the world had been forced into an uneasy alliance against overwhelming odds.

In Britain and Germany, caches of once-forbidden weapons were unearthed, and unthinkable tactics were employed against the enemy. Even as lack of fuel forced people back to horses and carriages, physicists worked feverishly to create humanity's first nuclear bombs--with horrifying results. City after city joined the atomic pyre as the planet erupted in fiery ruins. The tactics of daredevil guerrillas everywhere became increasingly ingenious against a superior foe whose desperate retaliation would grow ever more fearsome.

No one had ever put the United States, or the world, in such deadly danger. But if the carnage and annihilation ever stopped, would there be any pieces to pick up?