The Dark of the Sun By Wilbur Smith

Bruce Curry sets out with a trainload of mercenaries to relieve a mining town besieged by rebels in the heart of the African jungle. The journey, turns into a nightmare, softened only by Shermaine, a Belgian girl he meets and with whom he falls in love. 

But love is no elixir. In fact, it considerably complicates Curry's struggle to carry out his mission and stay alive. He finds himself buffeted by emotions he thought would never visit him again, and he notes with wry amusement his own distraction