Poland was annexed and partitioned by Germany and the Soviet Union in the aftermath of the invasion of Poland in 1939. In the pre-war Polish territories annexed by the Soviets (Western Ukraine, Western Belarus, Lithuania and Białystok regions, known to Poles as "Kresy") the first Soviet partisan groups were formed in 1941, soon after Operation Barbarossa, the German invasion of the Soviet Union. Those groups fought against the Germans, but conflicts with Polish partisans were also common.