Development and Change of the Northern Borders in Times of the Sui and the Tang Dynasty

The Tang Dynasty's and northern peoples headed by Tujue's respective territories and actual control areas went through dozens of change in their struggles and intercourses.The early Sui Dynasty used to come into an agreement with Tujue of bordering by the and extend its northern border to the south of the desert.However,since Shatilue Khan,Tujue started to expand into the south of desert in disguise of lodging at the Baidao Valley as a regime under the Sui Dynasty and gradually brought the whole desert area under control.By the Imperial Title of Daye when Shibi Khan started to resist the Sui Dynasty openly,the Sui Dynasty was bordered with Tujue by the Bend of the Yellow River stretching from west to east in line along the Yellow River and Great Wall.In the late Sui and early Tang times,Tujue actually controlled most of counties north of the Guanzhong Plains for dozens of years either by supporting the anti-Sui forces or by occupying the territory in arms.After the fall of East Tujue,the Tang Dynasty expanded its northern border into farthest north in the reigns of Taizong and Gaozong but shrank back for a time in the reign of Empress of Wu.During the Imperial Titles of Kaiyuan and Tianbao in the reign of Xuanzong,the Tang Dynasty's northern border was basically ascertained around the Yin Mountains.