Two Hawks from Earth

The Gate of Time (1966), revised and expanded as Two Hawks from Earth (1979). Roger Two Hawks thought he'd probably bought it when he bailed out of his flaming bomber. His plane had made an eerie shudder just before he jumped, but nothing could have prepared him for what he found on the ground: Men dressed in skins, fighting with knives and arrows, speaking no language he'd ever heard. The War – and with it all the rest of the world he knew – had vanished, and been replaced by a savage struggle for control of a primitive parallel Earth. Two Hawks' technological know-how makes him a valuable prize for his captors – too valuable to be set free... and too dangerous for the other side to leave alive.