Orion #2: Vengeance of Orion by Ben Bova

Fresh from a space battle, Orion finds himself thrust back in time to confront gods in the ancient world, to Greece where he must prevent the Greek army from destroying the citadel of Troy, to Jericho to bring down the walls, to Egypt… If he fails, he will lose the only woman he has ever loved. But if he succeeds, the history of the world will be changed forever.