The territory of present day Ukraine in the 11-13th centuries was the center of the Kyivan Rus Kingdom, which is considered the polity from which the proto-Ukrainian society originated. The most prominent among the East Slavic tribes’ inhabiting these lands in the 9th century were Polianians settling on the banks of Dnipro River. The legend holds it that the Polianian leader Kyi founded a city on the river banks and named it after self. With time Kyiv became a center and a capital of a powerful medieval state of Rus. The Golden Age of Kyivan Rus coincided with the reign of prince Volodymyr the Great (980-1015), who baptized the Rus and his son Yaroslav the Wise (1019-1054), who created the first legal code in Eastern Europe “Ruska Pravda” (“Justice of Rus”).
The 9th century Kyiv ruler, Prince Oleg (as well as his successors) introduced an effort of consolidation of the nearest tribal principalities around Kyiv. Gradually, all the East-Slav tribes and many non-Slavic people fell under dominion of the Kyiv Prince by the end of the 10th century. Kyivan Rus spread from the Black Sea to the White Sea and from the Carpathians to the Volga River.
Kyiv princes of the 9-10th centuries cared mainly about strengthening the economic and political power of the state. During Princess Olga's reign (approximately 946), the first attempt was made to introduce Christianity. Diplomatic relations of the Old Rus State with the neighboring countries of Byzanthia and the German Empire intensified during the mid-10th century after the fall of Khozar's state. The military marches of Kyiv Princes played an important part in the expansion of the territory of Kyiv Rus and assertion of its power. The “Povist Vremyanykh Lit” mentions the victorious raid of Prince Oleg of Tsarhorod in 907, which made peace with the Byzantine Emperor. In the 940s, Prince Ihor (Oleg's successor) made several military raids to the Crimean East, Taman, Byzanthia and to the Caspian Seaside. Military activity of the Old Rus State was also observed in the 960s and early 970s during the reign of Prince Sviatoslav (964-972).
The pinnacle of Kyivan Rus power and glory was achieved during the reign of Prince Sviatoslav's son, Prince Volodymyr (980-1015). His reign was marked by a rise of the economical and political strength of the state and the authority of the Kyiv Prince's rule. The successful military raids of the Prince expanded the limits of the Rus territory. The process of forming the Rus State finished in the beginning of the 11th century under Yaroslav the Wise. This was the time of Kyivan Rus glory. The international authority of the country increased, due to the dynastic relations and diplomacy of the Prince. Under Yaroslav the importance of cities in economic and cultural life of the state increased and relations between the different regions became vivid. This helped to increase the trade, agriculture and handicraft industries. The first code of the Old Rus state was created as a collection of laws known as “Ruska pravda.”
Under the reign of Volodymyr Monomakh (1113-1125) the authority of the Kyiv Prince expanded to the major Rus’ principalities. This was the period when all the characteristics of the medieval socio-political system with great feudal property, certain ideological, religious and political directions were established in Kyiv Rus.
From the 1130s the disintegration process of the Old Rus State, due to the frequent feuds among its many independent principalities, became irreversible. For several years the territory of this newly powerful state were separated into several independent principalities. Although, the authority of the Kyiv Prince as the state head formaly did not lead to the complete disintegration of the Old Rus state. The personal power of the Kyiv Prince was replaced by the government of "collective suzerainty" – the most influential and powerful group of Princes. A single centralized monarchy was changed into a federal monarchy, which no longer had the power of its predecessor.
The period of feudal disintegration on the Kyivan Rus lands not only left a mark on their political, socio-economic and cultural development, but also introduced certain innovations to the geographical definitions of the state. In particular, the Kyiv Chronicle of 1187 had first coined the term “Ukraine” to define the southern area of Rus lands (Kyiv, Pereyaslav and Chernihiv provinces). After some time, this name was also applied to Galicia, Volhynia, and Podillya. Despite several attempts to unite principalities separated by boundaries, Kyivan Rus of 1237 was weakened economically and politically and suffered the devastating forays of Mongol-Tatar hordes.
After the Kyivan Rus collapse in 1199, the established Kingdom of Galicia-Volhynia emerged as its powerful successor in the area of contemporary Western Ukraine.The Galicia-Volhynia ruler, Prince Roman was the first to be referred to as the “King of the Rus” in the history of the Old Rus state. He was crowned in 1253 by a papal representative in the town of Dorohychyn. The kingdom existed till 1349 when it eventually fell under the Polish and Lithuanian onslaught.
The movement of Lithuania and Poland to Ukrainian lands began in the 1330s and 1340s when the Lithuanian Grand Duke Liubard conquered Volhynia. The divide of the lands of the Galicia-Volhynia principality between two powers was ended by the Lithuanian-Polish War of 1351-1352, after which Galicia fell under the power of the Polish King (later the west Volhynian lands came under the Polish crown). The Podillya, Kyiv, Siver and Pereyaslav provinces also became parts of Lithuania under Grand Duke Olgerd's rule.