David Drake is, of course, one of the masters of SF, and this novel takes one of the classic ideas and runs with it. What if . . .?
From back cover:
The aliens were looking for a few good fighters, warriors who could be brave without blasters, and content with shields of bronze . . . What they got were ROMANS!
Gaius Vibulenas--from a raw kid, he became an accomplished soldier and a leader of men
Quartilla--She was everything a man could want---you'd never guess she wasn't human
Pompilius Niger--He learned young that the worst dangers a soldier faces come from those who give him his orders
Clidius Afer--a veteran, he didn't ask questions--until his alien masters forced the answers on him
The aliens knew that in these primitives they possessed the finest low-tech soldiers in the galaxy. What they did not understand was the madness of enslaving a Roman legion and leaving them with swords in their hands.