End of the Beginning by Harry Turtledove

Six weeks ago, Imperial Japanese military forces conquered and occupied the Hawaiian Islands. A puppet king sits on Hawaii's throne, his strings controlled by the general of the invasion force. American POWs, malnourished and weak, are enslaved as hard laborers until death takes them. Civilians fare little better, struggling to survive on dwindling resources. And families of Japanese origin find their loyalties divided.
Meanwhile, across the United States, from Pensacola, Florida, to San Diego, California, the military is marshaling its forces. Steel factories and fuel refineries are operating around the clock. New recruits are enlisting and undergoing rigorous training exercises---all for the opportunity to strike back and drive the enemy from American soil...