From the 13th century on, the Lakeland was successively part of the monastic state of the Teutonic Knights, the Duchy of Prussia, and the Prussian province of East Prussia. In modern times, while part of the German Empire, it was the location of the First Battle of the Masurian Lakes (1914) and the Second Battle of the Masurian Lakes (1915) during World War I. At the end of World War II (1945), following the decision taken at the Potsdam Agreement, all national borders were redrawn, the area annexed by Communist Poland and its German residents expelled.