Complex sentence: According to a popular and well established theory, the medieval state of Kievan Rus was established by the Varangians in the 9th century as the first historically recorded East Slavic state which emerged as a powerful nation in the Middle Ages until it disintegrated in the 12th century.
Split into simpler sentences: According to a popular and well established theory, the medieval state of Kievan Rus was established by the Varangians in the 9th century as the first historically recorded East Slavic state. It emerged as a powerful nation in the Middle Ages but disintegrated in the 12th century.