The Man with the Iron Heart by Harry Turtledove

What if V-E Day didn't end World War II in Europe? What if, instead, the Allies had to face a potent, even fanatical, postwar Nazi resistance? Such a movement, based in the fabled Alpine Redoubt, was in fact a real threat, ultimately neutralized by Germany's flagging resources and squabbling officials. But had SS Obergruppenfuhrer Reinhard Heydrich, the notorious Man with the Iron Heart, not been assassinated in 1942, fate might have taken a different turn. We might likely have seen a German guerrilla war launched against the conquerors, presaging by more than half a century the protracted conflict with an unrelenting enemy that now engulfs the United States and its allies in Iraq. How might today's clash of troops versus terrorists have played out in 1945? In this imagined world, Nazi forces resort to unconventional warfare, using the quick and dirty tactics of terrorism - booby traps, time bombs, mortar and rocket strikes in the night, assassinations, even kamikaze-style suicide attacks - to overturn what seemed to be a decisive Allied victory.