Ranks of Bronze/David Drake (1986)

An alien merchant empire acquires a legion of Roman soldiers, the best soldiers who ever lived, to fight its battles on low-technology worlds, as dictated by their civil leadership.  The Romans survived the historical massacre of Crassus in the eastern reaches of the Empire.

Gaius Vibulenas is a 17-year-old tribune who fights an unending series of battles for the alien masters. With Clodious Afer, his grizzled veteran noncommissioned officer, they prove themselves time and again, even as their numbers are slowly reduced, even with the incredible efforts of the alien technology that seems to resurrect the dead.

Eventually, they get some ideas about going home.

Drake uses Crassus' defeat in the eastern frontier in an ill-conceived battle as the source of his soldiers.  The richest man in Rome, Crassus defeated Spartacus and assumesd rule of Rome with Pompey and Ceasar.  In an effort to address his lack of military credentials, he undertook the expedition that led to his death and the destruction of a great Roman army.  It's from that battlefield that the legion is taken.

Given drugs to extend their lives, the Romans sleep between landfalls, when they are dispatched to do what they do best. 

Readers of combat SF will find this book quite exciting, certainly action-filled, although to be honest it turned out to be not entirely to my taste.  No slight to David Drake.  He has plenty of credits to indicate his skills at combat SF, including the classic Hammer's Slammers.  I did find the Roman history references interesting, which motivated me to get this book.  And the concept is quite appealing.

It's out of print, I'm sure.  Check Half.com or Amazon.