A fierce national East Prussia-related conflict between Germans and Poles after the First World War basically contrasted with the prewar situation in the province. After the decision taken at the Paris Peace Conference in 1919 to hold a plebiscite in two governmental districts of this German province on their inhabitants’ political affiliations, the vast population there had to take a test on the basic choice of their political, and simultaneously cultural, orientation. Today, researchers agree that the plebiscite of 1920 caused irreversible damage to the multiethnic area. There is no doubt that the so-called Ostdeutscher Heimatdienst organisation strongly contributed to this. The article raises questions as to what circumstances promoted the establishment of the organisation, who its principal actors were, and how they affected the East Prussian population.