In the year 882, it was stated in old chronicles that Oleg, the Prince of Novhorod, having killed Prince Askold and Prince Dir, mounted the Kyiv throne. He became the ruler of Kyiv or Old Rus, the first state of Old Slavs, which soon turned into one of the greatest countries of Medieval Europe and which played an important part in political life on the continent. It also served as a certain protective barrier between European civilization and nomadic East. Kyiv became the capital of the state.
The poly-ethnical Old Rus state was a monarchical form of government. When he proclaimed Kyiv to be the political center of Rus, Prince Oleg (as well as his successors) were greatly concerned about the problem of consolidation of the nearest tribal principalities around Kyiv - the force of central state institutions being applied it its territory. All the East-Slav tribes and many non-Slav people were under dominion of the Kyiv Prince at the end of the 10th century. Kyiv Rus spread from the Black Sea to the White Sea, from the Carpathians to the Volga River. The vastness of the territory determined the availability (within limits) of certain language and cultural peculiarities - a potentiality of centrifugal tendencies being inevitable.
The Prince's armed forces played the role of the state elite in Kyiv Rus until the early 11th century. Elder men at arms served as the Prince's advisers in the most important state affairs and occupied all administrative and court posts. Under the reign of Yaroslav Mudriy (or Yaroslav the Wise) (1019-1054), they performed only military functions, while administrative and legislative staffs were subject to boyars (old tribal aristocrats by birth).
Kyiv princes of the 9-10th centuries cared mainly about strengthening the economic and political power of the state. They fortified cities, put in order legal proceedings and a fiscal system, and regulated the obligations of the dependent population. During Princess Olga's reign (approximately 946), the first attempt was made to expel paganism and replace it with Christianity. But Christianity wasn't officially introduced as a state religion in Rus until 988 by Prince Volodymyr Sviatoslavych. Diplomatic relations of the Old Rus State with the neighboring countries, in particular, 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 assertions of its power in the eyes of surrounding people. The "Povist mynylykh lit" mentions the victorious raid of Prince Oleg of Tsarhorod in 907, owing to having made peace with the Byzantine Emperor. Some years later, the Russians made several raids on the lands of the Arabian caliphate. In the 940s, Prince Ihor (Oleg's successor), made several military raids to the Crimean East and Taman, to 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 creation of the Old Rus nation state took place during the reign of Prince Sviatoslav's son, Prince Volodymyr (978-1015). The economical and political strength of the state, the authority of the Prince's rule, and the organization of law considerably increased during his reign. The successful military raids of the Prince expanded the limits of the Rus territory.
The process of forming the Old Rus State finished in the beginning of the 11th century under Yaroslav Mudriy. That was the time of the greatest rise of Kyiv Rus. The international authority of the country increased, due to the dynastic relations and diplomacy of the Prince. Yaroslav put forth much effort to subdue civil war (which occurred after the death of Volodymyr) and to protect the state territory from nomad raids. Under Yaroslav the importance of cities in economic and cultural life of the state increased, and relations between the different regions became revived, which helped to increase the trade, agriculture and handicraft industries. The first code of the Old Rus state was created - a collection of laws, "Ruska pravda". Unfortunately, the Prince's successors were involved in many feuds that inevitably resulted in breaking the unity of the Rus state.
It wasn't until the early 12th century that Volodymyr Monomakh (1113-1125) managed to stop these feuds for a while. It was under his reign that Kyiv's authority as the capital was once again increased, and the authority of the Kyiv Prince expanded to the major principalities, and other princes. It was by his initiative that the convention of princes was called to decide important affairs and disputable issues. The internal and external position of the state was stabilized. This was the stage when all the characteristics of the medieval socio-political system with great feudal property, certain ideological religious and political directions had been established in Kyiv Rus.
From the 1130s the disintegration process of the Old Rus State attained an irreversible character. For several years, the territory of this newly powerful state was separated into several independent principalities whose owners did not stop military conflicts until the mid-13th century. The authority of the Kyiv Prince as the state head became quite formal but did not lead to the complete disintegration of the Old Rus state. Kyiv still remained its capital. The personal power of the Kyiv Prince was replaced by the government of "collective suzerainty" of the most influential and powerful Princes. A single centralized monarchy was changed into a federal monarchy, which no longer had the might nor size of its predecessor.
The period of feudal disintegration on the Old Rus lands not only set a mark
on their political, socio-economic and cultural development, but also
introduced certain innovations to 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, Pereiaslav and Chernihiv
provinces). After some time, this name was also applied to Halychyna,
Volyn, and Podillia. Despite several attempts to unite principalities
separated by boundaries, which took place during the 12th and 13th centuries,
Kyiv Rus of 1237 weakened economically and politically and suffered the
forays of Mongol-Tatar Hordes of Batyi. The Horde reign in the lands of
Ukraine continued for more than two centuries.