The Einsatzgruppen Mobile Killing Units
From the mid-1300's, Jews had begun to concentrate in a large strip of eastern European territory known as the "Pale of the Settlement." By 1900, there were, perhaps, as many as 7 million Jews living in this area bounded by Germany on the east, the Baltic sea on the north, the Black Sea on the south and the Dnieper River in Russia on the east. The Jewish population of Poland in 1939 was about 3.3 million with an additional 2.1 million in the occupied Russian provinces. There were also heavy concentrations of Jews in Lithuania, Latvia and Estonia to the north and Hungary and the Slavic states to the south.
Anti-Semitism had long been evident in Poland. Jews were not considered Poles and, as in Nazi Germany, were defined as a race. It appears that, until 1939, Poland saw its destiny as tied to Germany's and its policies toward Jews mirrored those of Germany — forced emigration. This was all to change with the Nazi invasion of Poland on September 1, 1939. The consequences of this invasion were disastrous for Poland as a nation and, especially for Poland's Jewish population.
Immediately following the invasion, Heinrich Himmler was appointed to take measures to strengthen German ethnicity in the occupied territories and to create lebensraum, or living space for German citizens. To this end, Himmler created special task forces within the SS, the Einsatzgruppen, and placed them under the command of Reinhard Heydrich. On September, 21, 1939, Heydrich instructed those under his command to observe a distinction between the "final aim," which would take some time and "the steps necessary for reaching it which can be applied more or less at once." The Einsatzgruppen became "mobile killing units" charged with liquidating all political enemies of the Reich. According to historian, Raul Hilberg, the mobile killing units murdered 1.4 million Jews between 1941 and the end of the war in 1945.
Source: The Holocaust\Shoah Page.