This theme is dealt with in a lot of historical sources. The following citation is to illustrate this:
“During the Golden Age of Kiev, the lands of Rus' covered modern western, central and northern Ukraine, Belarus, and western Russia. But it did not include modern eastern and southern Ukraine, which were inhabited by nomads and had a different history.
In the 9th century, much of modern-day Ukraine was populated by the Slavic tribes. The so-called Kievan Rus was founded by Rus' people, Varangians who first settled around Ladoga and Novgorod, then gradually moved southward eventually reaching Kiev about 880. Kievan Rus' included the western part of modern Ukraine, Belarus, with larger part of it situated on the territory of modern Russia.
During the 10th and 11th centuries, it became the largest and most powerful state in Europe.In the following centuries, it laid the foundation for the national identity of Ukrainians and Russians. Kiev, the capital of modern Ukraine, became the most important city of the Rus'. According to the Primary Chronicle, the Rus' elite initially consisted of Varangians from Scandinavia.
The Varangians later became assimilated into the local Slavic population and became part of the Rus' first dynasty, the Rurik Dynasty. Kievan Rus' was composed of several principalities ruled by the interrelated Rurikid Princes. The seat of Kiev, the most prestigious and influential of all principalities, became the subject of many rivalries among Rurikids as the most valuable prize in their quest for power.
The Golden Age of Kievan Rus' began with the reign of Vladimir the Great (980–1015), who turned Rus' toward Byzantine Christianity. During the reign of his son, Yaroslav the Wise (1019–1054), Kievan Rus' reached the zenith of its cultural development and military power. This was followed by the state's increasing fragmentation as the relative importance of regional powers rose again. After a final resurgence under the rule of Vladimir Monomakh (1113–1125) and his son Mstislav (1125–1132), Kievan Rus' finally disintegrated into separate principalities following Mstislav's death. …”
In the 11th and 12th centuries, constant incursions by nomadic Turkic tribes, such as the Pechenegs and the Kipchaks, caused a massive migration of Slavic populations to the safer, heavily forested regions of the north. The 13th century Mongol invasion devastated Kievan Rus'. Kiev was totally destroyed in 1240. On the Ukrainian territory, the state of Kievan Rus' was succeeded by the principalities of Galich (Halych
) and Volodymyr-Volynskyi, which were merged into the state of Galicia-Volhynia. …”