The Timeline Wars — John Barnes

Blending alternate history, time travel, and detective thriller, the “able and impressive” Nebula and Hugo Award nominee gives us Nazis, Tories, and togas (The Washington Post).
 
An attack by time-hopping terrorists turns Mark Strang's life upside down, and the Pittsburgh art historian discovers his calling as a bodyguard for hire. Strap yourself in for a ride through alternate timelines in this action-packed series that combines sci-fi, time travel, alien invasion, and high-tech adventure.
 
Patton's Spaceship: When he learns the aliens behind the terrorists who destroyed his old life are trying to take control of timelines and subject them to totalitarian rule, Mark Strang will journey to a not-too-distant past to stop them -- and get revenge. But he lands in a timeline where America lost World War II. He can help the resistance with his knowledge of future technology, but is he permanently trapped in a Nazi-controlled past?
 
Washington's Dirigible: With the inhuman Closers still threatening timelines, Strang joins forces with their enemies, the ATN. He and time agent Chrysamen ja N'wook travel to an alternate colonial America to locate a missing operative and find that the colonies are on friendly terms with England and George Washington reigns as Duke of Kentucky. But he has one real enemy here -- himself.
 
Caesar's Bicycle: On assignment in ancient Rome, Mark Strang discovers the Closers have infiltrated the timeline and Julius Caesar is under their influence. Even as the Closers rewrite history to tip the scales in their favor, Strang is reluctant to assassinate an important government figure. But as he delays, his life -- and those of his companions -- hang in the balance, and they face a gruesome demise.