Janissaries, book 1
Jerry Pournelle1979

Some days it just didn'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...

The mercenaries are offered a deal, travel to the stars, to a primitive planet, and fight the enemies of the aliens. Intrigue, danger, and death await.