Shadow on the Stars — Robert Silverberg

A THRILLING ADVENTURE that ranges across time and space, pitting one man against a invading alien horde that threatens a colony world, and against a hidden conspiracy to conquer Earth. Blair Ewing must battle Klondi hordes, Sirian plotters, the forces of history, and the nature of time itself. It's a lot to ask of one man -- but supposing there were more than one? This classic early Silverberg novel folds an exploration of time-travel paradoxes into the story of one man's fight to save two worlds at once.