This study aims to outline changes over 300 years in the contact zones of the Tang dynasty (618-907). The empire is considered the most open dynasty in terms of interactions and exchanges with neighboring states than any other dynasties in the history of China. However, its openness does not mean that there were no contact zones separating the dynasty from the surrounding states. The frontier prefectures(Bianzhou) as the contact zones of Tang were different from the indirectly administered prefectures (Jimizhou), where the Chinese were indirectly involved with other ethnic peoples, and also different from the regular prefectures(Zhengzhou). The spatial structure of the contact zones was constructed complicatedly with military bases and installations such as the Great Wall, border forts and frontier junctures(Guansai), garrison towns(Jun), garrison stations(Zhen), signal fires , and post houses (Yi). Some Bianzhou, prefecture-level administrative districts among the Zhengzhou established in the frontiers, also included places for civilian settlement as well as the space for military use. The contact zones of Tang were characterized by a combination of these three concepts: points, lines and aspect.
The contact zones of Tang, depending on its war and peace with the surrounding states, were not fixed but continuously changed their spaces and borders. Although the Tang’s territorial expansion reached its zenith during the early imperial period, its borderlands were not expanded indefinitely. The northern contact zones between the Tang and the Türks did not move far beyond the great walls built in the Sui dynasty. In the Ordos, the Yellow River was perceived as a borderline, and the frontier only expanded partially to the northern part of the River by building the three cities for receiving surrenders (Sanshouxiangcheng) there in 709. In the northwest, the Tang expanded to the areas of the Hexi Corridor and the four garrison stations of Anxi , but in the west, the Tang’s expansion remained in the status quo as the Tibetan Empire had occupied the Tuyuhun and won a major victory against the Tang in the battle of Dafei River (Dafeichuan). Until the reign of Emperor Xuanzong of Tang, there were only a series of minor advances and retreats in the frontier areas where the Tuyuhun and the Qinghai resided.
After the An Shi Rebellion from 755-763, the Tang’s contact zones were reduced depending on changes in internal and external situations. In the external respect, the northern contact zones of Tang remained unchanged since the Uyghurs immediately succeeded the Tujue, who ruled from 692-745, in 742-840. On the other hand, in the west and northwest, a great change occurred as the war and settlement between the Tang and the Tibetans were repeated. Through the war and settlement, they reconfirmed some existing borderlands and treated some contact zones such as Yanzhou, Lingzhou (靈州) and the An Shi Rebellion as a means for deals and negotiations during the processes of readjustment, and even the Tibetans used some to expand their forces. After the An Shi Rebellion, the Tibetans had conquered 27 Bianzho including Yaozhou (姚州 754), Minzhou (岷州 758), Weizhou (維州 759), Lanzhou (蘭州 762), Hezhou (河州), Shanzhou (鄯州), Taozhou (洮州), Ganzhou (甘州 766), Suzhou (肅州 766). This means that more than half of the 50 Bianzhou established by Tang fell under the control of the Tibetans. Even the Anxisizhen in the west as well as the prefectures adjacent to the Tang’s capital city, Changan (長安), such as Qinzhou (秦州), Chengzhou (成州), Weizhou (渭州) and Yuanzhou (原州), which had been part of Zhengzhou during the early Tang period, came under the direct rule of the Tibetans.