Poul Anderson 
Time Patrol 
A Time Patrol Book

Forget minor hazards such as the H-Bomb. The discovery of time travel means that everything we know, anyone we know, might not only vanish, but never even have existed. Against that possibility stand the men and women of the Time Patrol, dedicated to preserving the history they know and protecting the future from fanatics, terrorists, and would-be dictators who would remold the shape of reality to suit their own purposes.Manse Everard, the Patrol’s finest temporal trouble-shooter, bears a heavy burden. The fabric of history is stained with human blood and suffering which he cannot, must not do anything to alleviate, lest his tampering bring disastrous alterations in future time. Everard must leave the horrors of the past in place, lest his tampering – or that of the Patrol’s opponents, the Exaltationists – erase all hope of a better future, and instead bring about a future filled with greater horrors than any recorded by past history at its darkest and most foul.