The Soviet and Belarusian historiographies study the subject of German occupation in the context of contemporary Belarus, regarded as the Byelorussian Soviet Socialist Republic (BSSR), a constituent republic of the Soviet Union in the 1941 borders as a whole. Polish historiography insists on special, even separate treatment for the East Lands of the Poland in the 1921 borders (alias "Kresy Wschodnie" alias West Belarus), which were incorporated into the BSSR after the Soviet invaded Poland on September 17, 1939. More than 100,000 people of different ethnic backgrounds, mostly Poles and Jews in West Belarus were imprisoned, executed or transported to the eastern USSR by Soviet authorities before the German invasion.