The Volga was known to the ancient Greeks as the Rha, but little was known about the river until the early Middle Ages, when Slavic tribes settled along its upper course, the Bulgars (see Bulgars, Eastern) along its middle course, and the Khazars in the south. Its importance as a trade route dates from that time. The Russians soon extended their control as far as Nizhny Novgorod, founded in 1221. The Mongol invasion of the 13th cent. resulted in the direct control by the Golden Horde of the Volga below Nizhny Novgorod and in the creation (15th cent.) of the Tatar khanates of Kazan and Astrakhan, which fell to Moscow only in the 16th cent. Sarai, on the Volga River near modern Volgograd, was the capital of the empire of the Golden Horde. The conquest of these territories was largely the work of the Cossacks, who used the Volga and its tributaries for their epic forays into Siberia (under Yermak in the 16th cent.) and into the Caspian Sea (under Stenka Razin, in the 17th cent.). Many of the Finnic and Turko-Tatar nationalities still live in the middle and lower Volga regions, notably in the Chuvash, Mari El, Mordovian, Tatar, and Udmurt republics. The Kalmyrs settled in the lower Volga region in the early 17th cent. The lower Volga was the center of the great peasant rebellions under Stenka Razin and Pugachev. After their suppression Catherine II settled many German colonists in the region around Saratov.