Lake district, northeastern Poland, containing more than 2,000 lakes. It extends 180 mi (290 km) eastward from the lower Vistula River to the Poland-Belarus border and occupies an area of 20,000 sq mi (52,000 sq km). It was the scene of Russian defeats in 1914-15 during World War I. It came under Russian control in January 1945 but was later granted to Poland by the Potsdam Conference