Up Country by Nelson DeMille

Having taken to the lifestyle of a middle-aged civilian, the last thing Paul Brenner wanted to do was return to work for the Army’s Criminal Investigation Division, the agency that thanked him for years of life-risking service by forcing him into early retirement. But when an old friend calls in a career’s worth of favours, Paul finds himself moonlighting for the Army as he investigates a puzzling murder that took place thirty years before in the midst of the Vietnam war. Forced to return to the country that haunts him and work for the people who cast him aside, Paul must engage in the battle of his life as he attempts to find justice in a world of staggering corruption.