The modern administrative-territorial structure of Russia is a system of territorial organization which is a product of a centuries-long evolution and reforms. Early history The Kievan Rus' as it formed in the 10th century remained a more or less unified realm under the rule of Yaroslav the Wise (d. 1054), but in the later part of the 11th century, it disintegrated into a number of de facto independent and rivaling principalities, the most important of which were Grand Duchy of Galicia and Volhynia, Novgorod Republic, and Grand Duchy of Vladimir and Suzdal. With the advance of Mongols and establishing of Golden Horde in 1240, many parts of Kievan Rus came under a direct administration of Sarai, while others became its dependencies. The three previously mentioned main centers were established as successors of the Kievan Rus. Most of Kingdom of Galicia–Volhynia however became part of the Grand Duchy of Lithuania and later gradually and completely coming under the direct administration of the Crown of Poland. Novgorod Republic was overran by the time well-established Grand Duchy of Moscow. The grand duchies of Lithuania and Moscow practically divided the former territories of Kievan Rus between each other, both struggling to gain the seat of Metropolitan of Kiev. From the 13th century, the Russian principalities used an administrative subdivision into uyezds, with each such uyezd being subdivided into several volosts, some areas used division of pyatina. Voivodes were the officials appointed to administer and defend the uyezds. By the 15th century, the Grand Duchy of Moscow was recognized as a direct successor of the Grand Duchy of Vladimir. It gradually incorporated all left out adjacent smaller duchies such as the Principality of Yaroslavl, Principality of Rostov and successfully conquered the Principality of Nizhny Novgorod-Suzdal, the Principality of Tver as well as the Novgorod Republic. Near the end of the 15th century the Golden Horde fell apart into several smaller khanates and Muscovy for the first time became a sovereign state. At the start of the 16th century, the Grand Duchy of Moscow managed to annex the Pskov Republic and conquer the Grand Duchy of Ryazan as well as secure number of territories that belonged to the Grand Duchy of Lithuania such as the Upper Oka Principalities and Sloboda Ukraine, thus extending its territory far south. In 1708, the Oka principalities and Sloboda Ukraine were incorporated into the first Kiev Governorate. During the second half