Please read David Drake's "Foreign Legions"
It's about Roman soldiers used by aliens to fight wars for them.
It is a series of related stories writen by Mr. Drake and others like David Weber, S.M. Sterling and David Flint. It's a very nice read.
Here is a short review.
Anyone who likes alternate history novels or enjoys reading anything pertaining to the Roman Empire, or both, will enjoy Foreign
Legions, a collection of novellas based on David Drake's 1975 short story -- and later novel -- Ranks of Bronze. Drake's original short
story begins the collection: As his fans already know, the story was based on an actual battle that occurred in 53 B.C., when Marcus
Crassus led his Roman army into Parthia and was soundly defeated. Crassus was killed, and his tens of thousands of legionnaires
were sold into slavery or used as mercenaries. In Drake's story, the legionnaires are sold to aliens, who use them as galactic
butt-kickers. It seems the aliens are part of a United Nations-type organization that has outlawed the use of modern technology against
primitive races. The aliens make the soldiers almost indestructible and use them to fight their wars.