The population of Poland decreased by about six million due to the losses sustained during the Holocaust and German occupation during World War II (1939-1945), and Poland became one of the most ethnically homogeneous areas in Europe (next to the populations of Slovakia and the Czech Republic) as a result of radically altered borders and population expulsions at the end of and after the war. The post-war population movements were accompanied by waves of forced migrations ordered by the Soviet and Polish communist authorities, including the transfers of sizable Polish populations (1944–1946) from Poland's pre-war eastern territories, which were incorporated into the Soviet Union, the expulsion of ethnic Ukrainians to the USSR (1944–1946), Operation Vistula (1947), and the expulsion of Germans (1945–1950) from former German provinces awarded to Poland.