The foreign relations of Tibet are documented from the 7th century onward, when Buddhism was introduced by missionaries from India and Nepal. The Tibetan Empire fought with the Tang dynasty for control over territory dozens of times, despite peace marriage twice. Tibet was conquered by the Mongol Empire and that changed its internal system of government, introducing the Dalai Lamas, as well as subjecting Tibet to political rule under the Yuan dynasty. Tibetan foreign relations during the Ming dynasty are opaque, with Tibet being either a tributary state or under full Chinese sovereignty. But by the 18th century, the Qing dynasty indisputably made Tibet a subject. In the early 20th century, after a successful invasion, Britain established a trading relationship with Tibet and was permitted limited diplomatic access to "Outer Tibet", basically Shigatse and Lhasa. Britain supported Tibetan autonomy under the 13th Dalai Lama but did not contest Chinese suzerainty; while "Inner Tibet", areas such as Amdo and Kham with mixed Chinese and Tibetan populations to the east and north, remained nominally under the control of the Republic of China although that control was seldom effective. Although the sovereignty of Tibet was unrecognized, Tibet was courted in unofficial visits from Nazi Germany, Imperial Japan, and the United States during and after World War II. The foreign relations of Tibet ended with the Seventeen Point Agreement that formalized Chinese sovereignty over most all of political Tibet in 1951. Early history Little is known of Tibet before the 7th century when Buddhism was introduced by missionaries from India; between the 8th and 10th centuries it was a strong empire. Relations with the Tang dynasty It is recorded in Tibetan tradition that after Songtsen Gampo died in A.D. 650, the Chinese Tang dynasty attacked and captured Lhasa, however there is no record of it from Chinese sources. There was a stone pillar, the Lhasa Zhol rdo-rings, in the ancient village of Zhol in front of the Potala in Lhasa, dating to c. 764 CE during the reign of Trisong Detsen. The inscription contains an account of the brief capture of Chang'an, the Chinese capital, in 763 CE during the reign of Emperor Daizong of Tang. As of 1993 the pillar was surrounded by buildings and wire so it could not be approached closely. In 785, Wei Kao, a Chinese serving as an official in Shuh repulsed Tibetan invasions of the area. A stone