The Second World War began in September 1939, when Hitler and Stalin invaded Poland, the Soviet Union taking most of Western Ukraine. Nazi Germany with its allies invaded the Soviet Union in 1941. Some Ukrainians initially regarded the Wehrmacht soldiers as liberators from Soviet rule, while others formed a partisan movement. Some elements of the Ukrainian nationalist underground formed a Ukrainian Insurgent Army that fought both Soviet forces and the Nazi. Others collaborated with the Germans. Some 1.5 million Jews were murdered by the Nazis during their occupation. In Volhynia, Ukrainian fighters committed a [[Massacres of Poles in Volhynia and Eastern Galicia|massacre]] against up to 100,000 Polish civilians. Residual small groups of the UPA-partizans acted near the Polish and Soviet border as long as to the 1950s.