'Tannenberg' was the German basic plan for extermination activities directed at the Polish people as part of the 'Generalplan Ost' (May/October 1939).

A list was drawn up with the names of more than 61,000 Polish activists, members of the intelligentsia, actors, former officers, etc for arrest and internment, or for execution, after the German conquest of Poland. Members of the German minority living in Poland assisted in preparing the list. The plan was created in May 1939. At the personal order of Adolf Hitler, a special unit dubbed 'Tannenberg' was created within the Reichssicherheitshauptamt (State Security Main Office), and had under command a number of Einsatzgruppen der Sicherheitspolizei staffed by Sicherheitspolizei and Sicherheitsdienst officers, who were theoretically subordinate to local German army commanders. Their task was to undertake the systematic arrest of all the people listed on the proscription lists prepared before the outbreak of war.

First, in August 1939 about 2,000 activists of Polish minority organisations in Germany were arrested and murdered. The second part of the action started on 1 September of the same year and ended in October, resulting in the murder of at least 20,000 persons in 760 mass executions by the Einsatzgruppen (special units) in addition to regular army and SS units. In addition to these a special formation, the Selbschütz, had been created from members of the German minority living in Poland. Its members trained in Germany before the war in diversion operations and guerrilla fighting. The formation was responsible for many massacres and acquired so appalling a reputation that it was dissolved by the German authorities after the September 1939 campaign which conquered Poland.