The Flying Warlord by Leo Frankowski

UNCERTAIN VISION

For a twentieth-century man, Conrad Stargard had done a lot in thirteenth-century Poland. In just nine years, he had "discovered" universal education, aircraft, radios, steamboats, and machine guns. More important, he had prepared Poland to defeat the bloodthirsty Mongols in 1241.

But now that the Mongol hordes had arrived, something was disturbing the flow of history. Even the Time Masters who secretly watched over Conrad couldn't predict his future, because they no longer even knew their own past . . .