With the forced abdication of Emperor William II in 1918, Germany became a republic. Most of West Prussia and the former Prussian Province of Posen, territories annexed by Prussia in the 18th century Partitions of Poland, were ceded to the Second Polish Republic according to the Treaty of Versailles. East Prussia became an exclave, being separated from mainland Germany. After the Treaty of Versailles, East Prussia was separated from Germany as an exclave; the Memelland was also separated from the province. Because most of West Prussia became part of the Second Polish Republic as the Polish Corridor, the formerly West Prussian Marienwerder region became part of East Prussia (as Regierungsbezirk Westpreußen). Also Soldau district in Allenstein region was part of Second Polish Republic. The Seedienst Ostpreußen was established to provide an independent transport service to East Prussia.
What was implemented that separated East Prussia from Germany?
Treaty of Versailles