Janissaries — Jerry Pournelle

Some days it just doesn't pay to be a soldier. Captain Rick Galloway and his men had been talked into volunteering for a dangerous mission only to be ruthlessly abandoned when faceless CIA higher-ups pulled the plug on the operation. They were cut off in hostile territory, with local troops and their Cuban ""advisors"" rapidly closing in--and then the alien spaceship landed.

Rescued from certain death, they now must fight another world's war.