The Dogs of War (1980) on Netflix

Jamie Shannon is a soldier of fortune -- a mercenary who will stage a coup or a revolution for the right price. He is hired by British mining interests to scout out Zangaro, a small African nation with rich mineral deposits but a brutal and xenophobic dictatorship. Arrested soon after his arrival, Shannon is imprisoned as a spy, badly beaten, and tortured. While in prison he meets one of the country's leading intellectuals, Dr. Okoye, also imprisoned by the regime. Eventually released, he returns to London and is subsequently offered to opportunity to secretly invade Zangaro's capital and lead a military coup. Shannon accepts, but quietly has his own agenda to pursue.