The Higuchian series Series by John Dalmas

One thousand years after World War III, a group of lizards attempts to take over what remains of Earth, a seemingly easy task considering the primitive levels of technology of the remaining humans.

After a global war crippled Earth's civilization, lizard-like aliens thought they would have no trouble taking over. Fortunately, secret galactic agents had trained the mystic warriors of Earth, and the aliens had been driven off. Now the Helverti are planning to succeed where the lizards failed.