 Upon the death of his uncle Frederick I, who had died without male issue in 1017, he inherited Eilenburg and Brehna.In 1018, Theodoric and his brother-in-law, Margrave Herman I of Meissen acted as witnesses when the Peace of Bautzen was concluded between Emperor Henry II and the Polish ruler Bolesław I the Brave, ending a lengthy German–Polish War.From 1029, Emperor Conrad II again waged war against Poland.