Masuria (məzŏˈrēə) [key], Ger. Masurenland, Pol. Mazury, region, N Poland. It is a low-lying area covered by large lakes and forests and drained by many small rivers. The original population of the region was expelled by the Teutonic Knights and replaced (14th cent.) with Polish settlers. Masuria later became part of East Prussia and was largely Germanized by the early 20th cent. After Masuria passed to Poland in 1945, most of the German-speaking population was expelled and replaced by Poles. The Masurian Lakes region, where more than 2,700 lakes are located, was the scene of heavy fighting early in World War I. Two Russian armies, commanded by generals Samsonov and Rennenkampf, were defeated in the region—Samsonov by Hindenburg at Tannenburg (Aug., 1914) and Rennenkampf by Mackensen in the lake country (Sept., 1914). The Russians were also repulsed (Feb., 1915) in Masuria in the so-called Winter Battle.