I haven't read the book. From what I understand the premise is that the Nazis developed the atomic bomb and were able to use that. Supposedly, Washington was nuked and America essentially surrendered to avoid annihilation. From the series it looks as though Americans with German backgrounds were then appointed to police the occupation, which I suppose is theoretically possible. Japan only really occupied the West Coast, with a large lawless expanse in the Rocky Mountains acting as a buffer between the two empires.

Parts of it are a stretch of the imagination, but it's certainly more plausible than Walking Dead, the popularity of which completely eludes me because of the idiocy of the premise, the incompetency of the explanation of the zombies and the utter inability of society to retake control.

After all, resistance would be difficult if the would-be rebels know that entire cities will simply be wiped off the face of the Earth as a response. The known ruthlessness of the Nazi and Japanese empires would help reinforce a sense of a helplessness.