Upon the death of Kievan Rus' ruler, Prince Yaroslav the Wise, the state split into independent principalities. These Ruthenian principalities were badly affected by a Mongol invasion in the 13th century, and many were later incorporated into the Grand Duchy of Lithuania. Of all the principalities held by the Duchy, nine were settled by ancestors of the Belarusian people. 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 border lands of Eastern Europe