The Proteus Operation

Utopia was achieved in the 21st century. But even in a world of prosperity and freedom there were malcontents. They called themselves Overlord, and their possession of time travel gave them the ability to change the past and shape the future according to their twisted desires. They built a time gate, then picked a crucial turning point in the previous century and traveled back to transmute an obscure figure on the lunatic fringe of German politics into an invincible conqueror. His name: Adolph Hitler. By the 1970s, only North America and Australia remained free, but time was running out and both would soon be faced with a choice of surrender or nuclear annihilation.

(From Goodreads)