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.