The Time Kings

They came out of the long grey ages of Time, killing and plundering, and their object was to take back captives to appease the blood-thirsty mobs. The city was burning when Paul Sanders drove into it that wet and foggy evening, the streets filled with tall, cruel-faced warriors. Taken prisoner by them, he and a handful of others found themselves transported in Time to the world of a million years hence.
A strange world filled with the weirdest anachronisms where a superstitious people were held in thrall by three men, the Time Kings, who alone knew the secret of time travel. The ancient knowledge had been destroyed by a people smarting under utter defeat. During the long ages there had been three Interstellar Empires when Earth had reigned supreme throughout the galaxy, but it remained for a group of men from the early dawn of the Atomic Age to overthrow the anarchy which prevailed and to restore Earth to her previous position of greatness, leading the people back to the stars which were their destiny.