The Lizard War — John Dalmas

It's a thousand years after World War III and Earth lies supine beneath the heel of a gang of alien sociopaths (they look like snakes with legs) who like to torture whole populations for sport. The Lizards could not have conquered us in our heyday, but our heyday was long gone when they stumbled upon us; the 16th century level of technology they found here was relatively easy to squelch.

What was not so easy to squelch was the mystic warrior sects that had evolved in the meantime. What should have been a simple mopping up operation to pacify the wilderness becomes -- THE LIZARD WAR