Much of Belarus (formerly the Belorussian Soviet Socialist Republic of the USSR, and then Byelorussia) is a hilly lowland with forests, swamps, and numerous rivers and lakes. There are wide rivers emptying into the Baltic and Black seas. Its forests cover over one-third of the land and its peat marshes are a valuable natural resource. The largest lake is Narach, 31 sq mi (79.6 sq km).
In the 5th century A.D. , Belarus (also known as White Russia) was colonized by east Slavic tribes. Kiev dominated it from the 9th to 12th century. After the destruction of Kiev by the Mongols in the 13th century, the territory was conquered by the dukes of Lithuania, although it retained a degree of autonomy. Belarus became part of the Grand Duchy of Lithuania, which merged with Poland in 1569. Following the partitions of Poland in 1772, 1793, and 1795, in which Poland was divided among Russia, Prussia, and Austria, Belarus became part of the Russian empire.
Following World War I, Belarus proclaimed itself a republic, only to find itself occupied by the Red Army soon after its March 1918 announcement. The Polish-Soviet War of 1918–1921 was fought to decide the fate of Belarus. West Belarus was ceded to Poland; the larger eastern part formed the Belorussian SSR, and was then joined to the USSR in 1922. In 1939, the Soviet Union took back West Belarus from Poland under the secret protocol of the Nazi-Soviet Nonaggression Pact and incorporated it into the Belorussian Soviet Socialist Republic. Occupied by the Nazis in World War II, Belarus was one of the war's most devastated battlefields.
When the Chernobyl nuclear power plant in Ukraine exploded in 1986, 70% of its radioactive fallout fell on the Belorussian SSR. Cancer and other illnesses have multiplied as a result.