Operation Tannenberg (Unternehmen Tannenberg) was the codename for one of the extermination actions directed at the Polish people during World War II, part of the Generalplan Ost. Conscription lists (Sonderfahndungsbuch Polen) identified more than 61,000 Polish activists, intelligentsia, actors, former officers, and others, who were to be interned or shot. Members of the German minority living in Poland assisted in preparing the lists.
The plan was created in May 1939. Following the orders of Adolf Hitler, a special unit dubbed Tannenberg was created within the Reich Security Main Office (Reichssicherheitshauptamt). It commanded a number of Einsatzgruppen der Sicherheitspolizei und des SD formed of Gestapo, Kripo and SD officers who were theoretically subordinate to local Wehrmacht commanders. Their task was to arrest all the people listed on the proscription lists prepared before the outbreak of World War II.
First, in August 1939 about 2,000 activists of Polish minority organisations in Germany were arrested and murdered. The second part of the action began on September 1, 1939, and ended in October, resulting in at least 20,000 deaths in 760 mass executions by special units or Einsatzgruppen (with some help from regular Wehrmacht units). In addition, a special formation was created from the German minority living in Poland called Selbstschutz, whose members had trained in Germany before the war in diversion and guerilla fighting. The formation was responsible for many massacres and due to its bad reputation was dissolved by Nazi authorities after the September Campaign.