The Sejny Uprising or Seinai Revolt (, ) refers to a Polish uprising against the Lithuanian authorities in August 1919 in the ethnically mixed area surrounding the town of Sejny (). When German forces, which occupied the territory during World War I, retreated from the area in May 1919, they turned over administration to the Lithuanians. Trying to prevent an armed conflict between Poland and Lithuania, the Entente drew a demarcation line, known as the Foch Line. The line assigned much of the disputed Suwałki (Suvalkai) Region to Poland and required the Lithuanian Army to retreat. While the Lithuanians retreated from some areas, they refused to leave Sejny (Seinai), because of its major Lithuanian population. Polish irregular forces began the uprising on August 23, 1919, and soon received support from the regular Polish Army. After several military skirmishes, Polish forces secured Sejny and the Lithuanians retreated behind the Foch Line. The uprising did not solve the larger border conflict between Poland and Lithuania over the ethnically mixed Suwałki Region. Both sides complained about each other's repressive measures. The conflict intensified in 1920, causing military skirmishes of the Polish–Lithuanian War. Sejny changed hands frequently until the Suwałki Agreement of October 1920, which left Sejny on the Polish side. The uprising undermined the plans of Polish leader Józef Piłsudski who was planning a coup d'état in Lithuania to replace the Lithuanian government with a pro-Polish cabinet which would agree to a union with Poland (the proposed Międzymorze federation). Because the Sejny Uprising had prompted the Lithuanian intelligence to intensify its investigations of Polish activities in Lithuania, they discovered plans for the coup and prevented it, arresting Polish sympathizers. These hostilities in Sejny further strained the Polish–Lithuanian relations. Eventually, Poland and Lithuania reached an agreement on a new border that left Sejny on the Polish side of the border. The Polish–Lithuanian border in the Suwałki Region has remained the same since then (with the exception of the World War II period). Background During the ages, the lands surrounding the town of Suwałki were part of the Grand Duchy of Lithuania until 1795. Sejny itself was property of Vilnius' Dominican friars from 1603 until 1805. During the Third Partition of the Polish-Lithuanian Commonwealth in 1795, the region became part of the Kingdom of Prussia as New East Prussia until 1807, from then until 1815, it was part of the Duchy of Warsaw which Napoleon