On 30 August 1939, Germany gave Poland an ultimatum demanding control of the city, provoking the Danzig crisis and the invasion of Poland on 1 September. The city-state aided Nazi Germany during the invasion. The Free City of Danzig Police and militia fought in the Battle of Westerplatte and the attack on the Polish post office in Danzig. After the Polish campaign, Danzig was annexed to Germany. The Nazis subjected Danzig's Jewish and Polish minorities to the violence of the Holocaust and the Nazi crimes against the Polish nation. The Red Army occupied Danzig on 30 March 1945 and expelled its Germans; an estimated 290,800 had fled or been expelled from the city by 1950. Gdańsk became part of Poland after the war.