Foreign Legions

Lots of soldiers work for civilians they don't like, but these Romans had it worse than most - their commanders were blue-skinned aliens! The guilds of star-traveling merchants had strict rules to prevent their technology from falling into the hands of the natives of planets they were exploiting: military operations had to be carried out with weaponry no more complex than swords and bows. That was no handicap to merchant princes with a galaxy to scour for military slaves to do their fighting for them. Some came to Earth for soldiers and returned to the stars with the best the planet had to offer. For over two thousand years the aliens thought they'd succeeded brilliantly - but then things changed! (from the front flap of the Baen Books first edition)