Following the German invasion of 1939, Greater Poland was incorporated into Nazi Germany, becoming the province called Reichsgau Posen, later Reichsgau Wartheland ("Warthe" being the German name for the Warta river). The Polish population was oppressed, with many former officials and others considered potential enemies by the Nazis being imprisoned or executed, including at the notorious Fort VII concentration camp in Poznań. Poznań was declared a stronghold city "(Festung)" in the closing stages of the war, being taken by the Red Army in the Battle of Poznań, which ended on 22 February 1945.