The Russian Federation succeeded the Union of the Soviet Socialist Republics (USSR) in 1991. The heart of the Russian Federation is Russia. The mediaeval state of "Rus" rose in the ninth century. It adopted orthodox Christianity as its religion in the 10th century. In the 13th century, Rus was overrun by the Mongols and the Golden Horde.
Subsequently, the Grand Duchy of Moscow gradually expanded its territory by conquest, annexation and exploration. By the 18th century, the Russian Empire had become the third largest empire in history, stretching from Poland, in the West, to Alaska, in the East.