Imperium - Tapa dura

American Brion Bayard was abducted on the streets of Stockholm, and thrust into what he thought was a truck. He was relieved that his captors were very apologetic, and very British - until they began speaking about nations which Bayard had never heard of. They were from Earth, but not his Earth. There are countess parallel Earths, each different, where history has taken every possible turn, but mostly there are uninhabitable worlds, destroyed by the discovery and misuse of crosstime travel. The Earth of the Imperium is at war with another parallel Earth and Bayard can stop the war by replacing the ruler of the aggressor Earth-because the ruler is a parallel version of Bayard. But when Bayard went on his mission, things weren't quite that simple. And that was only the beginning of Bayard's adventures as he guarded his new home world both from internal enemies and invaders from the other side of time, becoming the staunchest and most resourceful defender of the Imperium.

Keith Laumer was a Captain in the US Air Force and later an officer in the Diplomatic Corps, serving all over the world, giving him a solid background both for his fast-moving Adventure stories and his satirical comedies of Retief, the galaxy's only two-fisted diplomat. He was renowned as one of the top writers of SF adventure.