Songtsen Gampo ascended the throne at the age of thirteen. To arrange an alliance with Nepal, he sent a minister there to arrange a marriage for him with the Princess Bhrikuti Devi (Lha-mo Khro-gnyer-can-ma). When she came to Tibet for the marriage, she brought with her a statue of the Buddha-figure Akshobhya.
It is unclear when Songtsen Gampo sent his minister Tonmi Sambhota to learn Sanskrit. He studied in Kashmir, from the tutors Lipikara and Devavidyasimha. When Tonmi Sambhota returned to Tibet, he developed a script for writing the Tibetan language, based on the Indian Brahmi and Gupta scripts. Consequently, he translated The Tough Mystery texts into Tibetan.
According to A. F. Rudolf Hoernle (Manuscript Remains of Buddhist Literature Found in Eastern Turkistan), the Tibetan script was developed primarily from the Khotanese adaptation of the Indian Upright Gupta script. This is inferred from the Tibetan and Khotanese scripts employing similar manners for indicating initial and long vowels and for placing vowels in the order of their alphabets. These manners differ significantly from those used in most other Indian-derived scripts.
Khotan (Li-yul) was a Buddhist kingdom on the Silk Route along the southwestern rim of the Tarim Basin, just north of western Tibet. Its people were of Iranian origin and its form of Buddhism derived from India. A trade route ran from Khotan to Tibet via Kashmir and therefore, as A. H. Francke asserts (“The Tibetan Alphabet,” Epigraphia India, vol. 11), it is not unreasonable that Tonmi Sambhota met and studied with a Khotanese tutor in Kashmir.
Songtsen Gampo now sought a similar alliance with China through a marriage with Princess Wencheng the daughter of the Tang Emperor Taizong (r. 627 – 650). This arrangement was delayed, however, because Thokiki, the ruler of the Tuyuhun Kingdom in the Kokonor region of northern Amdo, present-day Qinghai Province of China, was also seeking a marriage with the princess. The Tuyuhun had ruled this region from the beginning of the fourth century.
Songtsen Gampo was intent on building an extensive empire beyond Central Tibet, first to the north and the east. A long period of wars ensued, during which he conquered the Qiang, Bailan (sBa’i-lang), and Dangxian (Thang-shang) tribes. Now the ruler of a much greater realm, the Tibetan Emperor Songtsen Gampo asked the Chinese Emperor Taizong once more for his princess in marriage. When he was refused, Songtsen Gampo attacked the Chinese frontier province of Songzhou in present-day Sizhuan Province. Finally, he received the Chinese princess as his bride in 641. She brought with her to Tibet another Buddha image.
The Tibetan Emperor built two temples in the city of Kyishoe, later known as Lhasa. He built Potala Palace on top of red hill . To house the two Buddha images brought by his Nepali and Chinese wives. Ramoche Tsuglagkang was constructed for the Nepali statue and Rasa Trulnang Tsuglagkang (Ra-sa ‘phrul-snang tsug-lag-khang), later called the Jokang, for the Chinese one. For security reasons, the location of the two statues was interchanged during the next generation.
During this period, Songtsen Gampo further extended the Tibetan Empire to parts of northern Burma and, in 640, to Nepal as well. This was the origin of the Tibetan family clans in Nepal of Tsang, Lama, Sherpa, and Tamang. In 643, the Tibetan Empire further expanded as Legmi, more commonly known in Tibetan as Li Migkya, the last ruler of Zhang-zhung, submitted and Zhang-zhung became a vassal state.
Citing traditional Tibetan sources, Namkhai Norbu (Necklace of Gzi) relates that Songtsen Gampo’s initial relations with Zhang-zhung were peaceful. In fact, the Tibetan ruler’s first wife was King Li Migkya’s daughter Li Tigmen (Li Thig-dman), for whom he gave in exchange his sister as wife to the Zhang-zhung king. The Zhang-zhung princess brought with her to the Yarlung court many aspects of Bon culture. In 643, however, Songtsen Gampo attacked and conquered Zhang-zhung and had King Li Migkya killed.
Taking advantage of the good relations between Tibet and China, Songtsen Gampo, in 645, sent a request to the Tang Emperor and subsequently built a temple on Wutaishan (Ri-bo rtse-lnga), the five-peaked sacred mountain of the Buddha-figure Manjushri in present-day Shanxi Province.
In 648, the Chinese Emperor Taizong sent a good-will mission to the Indian Emperor Harsha (r. 606 – 647). When the mission arrived, Harsha had already passed away and had been succeeded by Arjuna, his minister. Arjuna was intolerant of Buddhism, and accordingly, had most of the Chinese mission killed. The survivors fled to Nepal and sought Tibetan help there. Subsequently, the Tibetan armies invaded and defeated Arjuna in Bihar. This defeat was not recorded, however, in Indian histories. Songtsen Gampo died shortly thereafter in 649.