The History of Tibet can be divided into four periods:
The Tsanpo's Period
This period starts from the reign of Nyatri Tsanpo, the first of the Tsanpos, in 127 B.C and ends in 842 A.D. at the death of Lang Dharma, the last of the Tsanpos. A Buddhist monk owing to Lang Dharma’s brutal persecution of Buddhism assassinated Lang Dharma. During Tsanpo period, some 42 Tsanpos ruled over Tibet and reach to its peak during the empire of Songtsan Gampo's rule. Songtsan Gampo was an outstanding ruler who unified Tibet and changed his capital to Lhasa. He also sent his minister Sambhota to India to study Sanskrit and promulgate a script for the Tibetan on his arrival to Tibet. Songtsan Gampo married Princess Wencheng of the tang Court of China and Pricess Bhrikuti Debi of Nepal and constructed the famous Potala Palace and Jokhang temple.
The Period of Decentralization
This period started in 842 A.D. the year of Lang Dharma's assassination, and end in about 1260 A.D. It ended when Pagpa, the Abbot of Sakya monastery, became a vassal of Kublai Khan, the first Emperor of the Yuan Dynasty. No elaborated details are available in the history about this period except that Tibet became decentralized into a number of petty principalities during this period.
The Period of Sakya, Pagdu, and Karmapa's Rule
This period commenced with Sakya's rule over Tibet, followed first by Pagdu's rule in Lhaoka and then by Karmara's rule in the Tsang region (Shigatse). The sakya period is important for the reason that Tibet officially became an inseparable part of China for was the time during this phase. This period lasted from 1260 A.D to 1642 A.D during which political powers centered in the three regions of Sakya, Pagdu, and tsang successively ruled over Tibet.
The Period of the Gandan Podrang's Administration
This was the period when the Dalai Lama ruled Tibet. It began in 1642 A.D. when the 5th Dalai Lama overtook the ruling power from the Tsang ruler. It basically ended in 1951 when Tibet was liberated and came to a complete end in 1959 when rebellion led by the Dalai Lama was appeased and the People's Government of the Tibet, Autonomous Region was set up.
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A simple guide of Tibet History