Abstract: After Tubo occupied the Tuyuhun’s former territory,tribes like the Tuyuhun,Tangut and Bailan remained independent,forming a “middle zone” between the two empires.During the Tianshou era,the Tangut’s mass surrender sparked a Tang-Tubo struggle for the Huanghe jiuqu(“黄河九曲”).In the Changan era,the Tang advanced westward into the Hexi jiuqu(“河西九曲”),largely due to the collapse of the Tibetan Gaer family and the subsequent defection of tribes in intermediary regions such as Tuyuhun.With the reversal of power dynamics,the Tang and Tubo concluded the Shenlong Alliance,which favored the Tang.The “river” in the agreement likely refers to the Hexi jiuqu section within present-day Gonghe,while “Baishui” corresponds to the upper reaches of the Huangshui River.Gaining control of Hexi jiuqu,the Tang achieved a success in Longyou comparable to recovering the Anxi sizhen(“安西四镇”). Thereafter,the Tang-Tubo conflict shifted westward from the Chiling and Shanzhou line to the Hexi jiuqu,where fierce battles ensued.