Kartli is the largest and most populated province of Eastern Georgia. It includes the Georgian capital, Tbilisi, and two other major cities, Gori and Rustavi. It is bordered by the mountain-range of the Greater Caucasus to the north, by the province of Kakheti to the east, by Azerbaijan and Armenia to the south, by Turkey and the Samtskhe-Javakheti region to the south-west and by the province of Imereti to the west. In the 3rd century BC the ancient Eastern Georgian Kingdom of Kartli, also known as Iberia, was established here. Its king declared Christianity as the official religion of Kartli in 337 AD. In the early Middle Ages, Kartli lost its political importance because of the struggle between the King and strong feudal rulers, as well as the aggression of the strong Persian Kingdom. Even so, in a way, it remained Georgia's leader because of the independence of its Church and culture from Byzantine influence. Kartli was part of the united Georgian Kingdom in the central Middle Ages. (Georgia was united at the beginning of the 11th century, but Tbilisi, Kartli's main city, was not liberated until 1122. Immediately afterwards, the Georgian capital moved from Kutaisi to Tbilisi.) After the disintegration of the united Kingdom in the 15th century, Kartli became an independent Kingdom, which suffered from frequent Persian invasions. In 1762, the Kingdom of Kartli was united with the neighboring Kingdom of Kakheti. This Kingdom too was soon weakened by the Persian aggression. In 1801 the Kingdom of Kartl-Kakheti was annexed to the Russian Empire.