Upon the German defeat in World War I, the area was ceded to the newly established Second Polish Republic according to the 1919 Treaty of Versailles, with Międzychód as the westernmost town in Poland, administratively located in the Poznań Voivodeship. In the 1930s, the local German minority helped create a list of local Poles intended for persecution in the event of the German invasion and occupation. The town was again annexed by Nazi Germany and incorporated into the "Reichsgau Wartheland" upon the 1939 Invasion of Poland, which started World War II. During the German occupation, Poles from Międzychód were among the victims of large massacres carried out in November 1939 in as part of the "Intelligenzaktion". The Germans carried out expulsions of Poles, whoses houses were then handed over to German colonists as part of the "Lebensraum" policy. Germany established and operated a Nazi prison in the town. In August 1944, the Germans carried out mass arrests of local members of the Home Army, the leading Polish underground resistance organization. Międzychód returned to Poland after World War II in 1945, while the German minority was expelled in accordance with the Potsdam Agreement.