In the 10th to 11th centuries Kievan Rus' became one of the largest and most prosperous states in Europe. The reigns of Vladimir the Great (980–1015) and his son Yaroslav I the Wise (1019–1054) constitute the Golden Age of Kiev, which saw the acceptance of Orthodox Christianity from Byzantium and the creation of the first East Slavic written legal code, the Russkaya Pravda.
In the 11th and 12th centuries, constant incursions by nomadic Turkic tribes, such as the Kipchaks and the Pechenegs, caused a massive migration of Slavic populations to the safer, heavily forested regions of the north, particularly to the area known as Zalesye.
The Baptism of Kievans, by Klavdy Lebedev.
The age of feudalism and decentralization had come, marked by constant in-fighting between members of the Rurik Dynasty that ruled Kievan Rus' collectively. Kiev's dominance waned, to the benefit of Vladimir-Suzdal in the north-east, Novgorod Republic in the north-west and Galicia-Volhynia in the south-west.
Ultimately Kievan Rus' disintegrated, with the final blow being the Mongol invasion of 1237–40, that resulted in the destruction of Kiev and the death of about half the population of Rus'. The invaders, later known as Tatars, formed the state of the Golden Horde, which pillaged the Russian principalities and ruled the southern and central expanses of Russia for over three centuries.
Galicia-Volhynia was eventually assimilated by the Polish-Lithuanian Commonwealth, while the Mongol-dominated Vladimir-Suzdal and Novgorod Republic, two regions on the periphery of Kiev, established the basis for the modern Russian nation. The Novgorod together with Pskov retained some degree of autonomy during the time of the Mongol yoke and were largely spared the atrocities that affected the rest of the country. Led by Prince Alexander Nevsky, Novgorodians repelled the invading Swedes in the Battle of the Neva in 1240, as well as the Germanic crusaders in the Battle of the Ice in 1242, breaking their attempts to colonize the Northern Rus'.