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In the late '70s or early '80s, I was looking at books at Tower Books - remember them? There was a sci-fi novel about mercenaries in Southern Africa who were in a firefight. They were outnumbered and had to flee. They ran into a craft which took off. Turned out to be piloted by aliens. The aliens either talked them or forced them to become mercenaries for them. May have some of the facts wrong.
Drove me nuts over the years. Would look for it at book stores but there are so many sci-fi novels. Does anyone remember this?
Word of advice - if a book looks interesting, buy it!