Yaroslav I "the Wise"
(artist unknown)

Kievan Rus achieved its greatest power and splendor under Yaroslav the Wise in the 11th century. Yaroslav made Kiev a great city and built magnificent buildings, including the notable Cathedral of Saint Sophia or Hagia Sophia of Kiev. Yaroslav did much to develop Kievan Rus education and culture. He also revised the first Russian law code, the so-called Russkaya Pravda or Russian Justice. After his death in 1054, Kievan Rus declined. Yaroslav's grandson, Vladimir II Monomachus, made the final attempt to unite Kievan Rus, but after his death in 1125 the fragmentation continued as other Kievan Rus principalities challenged Kiev's supremacy.

By the 13th century, the East Slavic lands became a loose federation of city-states, held together by common language, religion, traditions, and customs. Although ruled by members of the house of Rurik, these city-states were often at war with one another. The area became an easy target for bands of invading Asiatic Mongols.