The article discusses how the northern borders of the Sui and Tang Dynasties changed over time due to conflicts with the Tujue people. Initially, the Sui Dynasty controlled the northern border up to the desert, but Tujue gradually expanded southward. During the late Sui and early Tang periods, Tujue controlled many northern counties. The Tang Dynasty later expanded its northern border under Taizong and Gaozong but retreated during the reign of Empress of Wu. Eventually, in the reign of Xuanzong, the Tang Dynasty's northern border was stabilized around the Yin Mountains.