Complex sentence: 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.
Split into simpler sentences: During this time, the Duchy was involved in several military campaigns. Together with Poland was outfighted the Teutonic Knights at the Battle of Grunwald in 1410; the joint victory allowed the Duchy to control the northwestern borderlands of Eastern Europe.