Incorporation into the Grand Duchy of Lithuania resulted in an economic, political and ethno-cultural unification of Belarusian lands. Of the principalities held by the Duchy, nine of them were settled by a population that would eventually become the Belarusians. During this time, the Duchy was involved in several military campaigns, including fighting on the side of Poland against the Teutonic Knights at the Battle of Grunwald in 1410; the joint victory allowed the Duchy to control the northwestern borderlands of Eastern Europe.