Stettin belonged to Germany until 1945 as a part of the state of Prussia, province of Pommern. After WW II the German territories eastern the Oder-Neisse line were given under Polish administration. Notwithstanding Stettin is located western of the Oder-River it was given to Polish administration too, to give to Poland the whole of the Oder-mouth into Baltic Sea. The Polish drove away the Germans from their homes and settled Polish population there as they did in the other German territories eastern of the Oder river. So it is now a Polish town: Sczezin. By treaty along with German reunification 1990 Germany had to acknowledge its territorial losses to Poland.
The whole thing has to do with the western shift of Poland by Roosevelt, Stalin and Churchill, to leave to Russia the eastern territories of Poland (which in reality were Belorussian and Litowian)which they had got by the Hitler-Stalin-Treaty 1939 and to compensate Poland with German territories. For decades there had been disagreement between the Federal Republic of Germany (Western Germany) as to the "temporary" state of Polish administration of German territories, which Poland had always claimed as definitive.