Recall Not Earth
Mercenaries of the Galaxies.

All life on earth had been destroyed in a quick and easy kill by the great and ruthless intergalactic Vulmot Empire. Only an elite group of spacemen survived the holocaust. The last earthlings, they were scattered wanderers through the galaxies, /hiring out their fighting skills to whatever power needed them, yearly growing more bitter, dissolute, despairing.

By now the pitiful remnants of the Space Corps lacked even the will to fight—until a tantalizing promise from an enslaved race made them assemble one last time, for a mission only men with nothing to lose would attempt, for a final battle only desperate courage could win...