Complex sentence: During World War II, Lebus, including its medieval center, was almost completely destroyed, and its land east of the Oder joined Poland in 1945 according to the Potsdam Agreement.
Split into simpler sentences: During World War II, Lebus, including its medieval center, was almost completely destroyed. While Lebus remained part of East Germany, the area east of the Oder became part of Poland in 1945 according to the Potsdam Agreement.