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Kievan Rus' (Old East Slavic: Рѹ́сь, Рѹ́сьскаѧ землѧ, Greek: Ῥωσία, Latin: Rus(s)ia, Ruscia, Ruzzia, Rut(h)enia, Old Norse: Garðaríki) was a loose federation of East Slavic tribes in Europe from the late 9th to the mid-13th century, under the reign of the Rurik dynasty. The modern peoples of Belarus, Ukraine, and Russia, all claim Kievan Rus' as their cultural inheritance. At its greatest extent in the mid-11th century, it stretched from the Baltic Sea in the north to the Black Sea in the south and from the headwaters of the Vistula in the west to the Taman Peninsula in the east, uniting the majority of East Slavic tribes.
Kievan Rus' begins with the rule (882–912) of Prince Oleg I, who extended his control from Novgorod south along the Dnieper river valley in order to protect trade from Khazar incursions from the east and moved his capital to the more strategic Kiev. Sviatoslav I (died 972) achieved the first major expansion of Kievan Rus' territorial control, fighting a war of conquest against the Khazar Empire. Vladimir the Great (980–1015) introduced Christianity with his own baptism and, by decree, that of all the inhabitants of Kiev and beyond. Kievan Rus' reached its greatest extent under Yaroslav I (1019–1054); his sons assembled and issued its first written legal code, the Rus' Justice, shortly after his death.
The state declined beginning in the late 11th century and during the 12th century, disintegrating into various rival regional powers. It was further weakened by economic factors such as the collapse of Rus' commercial ties to Byzantium due to the decline of Constantinople and the accompanying diminution of trade routes through its territory. The state rose up, however, and was able to defend itself from the Mongol invasion of Rus' lands, with Mstislav III leading a large army to victory in the Battle of the Kalka River. After defeating the Mongols, he went on a campaign to unify the Slavic principalities under himself, becoming the Tsar and founding the Tsardom of Kiev.