After two civil wars, Vytautas the Great became the Grand Duke of Lithuania in 1392. During his reign, Lithuania reached the peak of its territorial expansion, centralization of the state began, and the Lithuanian nobility became increasingly prominent in state politics. In the great Battle of the Vorskla River in 1399, the combined forces of Tokhtamysh and Vytautas were defeated by the Mongols. Thanks to close cooperation, the armies of Lithuania and Poland achieved a victory over the Teutonic Knights in 1410 at the Battle of Grunwald, one of the largest battles of medieval Europe.