Complex sentence: After World War II, the devastated city along with all of Pomerania east of the Oder-Neisse line became Polish, and the remaining Germans either fled or were expelled.
Split into simpler sentences: After World War II, the devastated city along with all of Pomerania east of the Oder-Neisse line the region was placed under Polish administration and ethnically cleansed. The native German populace was the remaining Germans either fled or were expelled.