The name "Buriyat" is mentioned for the first time in a Mongolian work (1240). Consolidation of tribes and groups took place in the conditions of the Russian state. In addition to genuine Buryat-Mongolian tribes (Bugalat, Khora, Ekhirit, Khongodor) which merged with the Buryats, the Buryats assimilated also other groups, for example Oirots, Khalkha Mongols, Tungus (Evenks) and others. The territory/people were annexed to the Russian state by treaties in 1689 and 1728, when the territories on both the sides of Lake Baikal were separated from Mongolia. From the middle of the 17th c. to the beginning of the 20th c., the Buryat population increased from 27,700 to 300,000. The historical roots of the Buryat culture are related to the Mongolian one. After Buryatia was incorporated into Russia, it was exposed to two cultures - Christian and Buddhist traditions. Buryats west of Lake Baikal (Irkutsk Buryats) are "russified", and they soon abandoned nomadism for agriculture, whereas the eastern (Transbaykal) Buryats are closer to the Mongols, may live in yurts and are mostly Buddhists. In 1741, the Lamaist branch of Buddhism was recognized as one of the official religions in Russia, and the first Buryat datsan (Buddhist monastery) was built. The 2nd half of the 19th c. and the beginning of the 20th c. was a period of growth for the Buryat Buddhist church (48 datsans in Buryatia in 1914). Buddhism became an important factor in the cultural development of Buryatia. After the Revolution most of the lamas were loyal to the Soviet power. In 1925, a battle against religion and church in Buryatia started. Datsans were gradually closed down, and the activity of the church curtailed. Consequently, in the late 1930s, the Buddhist church ceased to exist and thousands of cultural treasures were destroyed. Attempts to revive the Buddhist Church started during World War II, officially it was re-established in 1946. Genuine revival of Buddhism has taken place since the late 1980s as an important factor in the national consolidation and spiritual rebirth. In 1923, Buryat-Mongol Autonomous Soviet Socialist Republic was formed and included Baykal province (Pribaykalskaya guberniya) with Russian population. In 1937, in an effort to disperse Buryats, Stalin's government separated a number of counties (rayony) from Buryat-Mongol ASSR and formed Ust-Orda Buryat Autonomous Okrug and Aga Buryat Autonomous Okrug; at the same time some counties with Buryat population were left out. Fearing Buryat nationalism, Stalin had more than 10,000 Buryats killed. In 1958, the name "Mongol" was taken away from the name of the republic (Buryat ASSR). BASSR declared its sovereignty in 1990, Republic of Buryatia since 1992, constitution of the Republic adopted by People's Hural in 1994, bilateral treaty with the Federation signed in 1995.