In 755, the general An Lushan rebelled against Tang, and by 756, the forces of his Yan dynasty were approaching the Tang capital Chang'an, forcing Xuanzong to flee to Jiannan Circuit (劍南, headquartered in modern Chengdu, Sichuan). Xuanzong's son and crown prince Li Heng fled to Lingwu and was declared emperor there. Emperor Suzong summoned forces from various border outposts, including from Anxi. Li Siye was then the deputy military governor of Anxi, and he initially suggested to the jiedushi Liang Zai () that they wait for more information. Duan Xiushi, however, rebuked him, pointing out that Suzong's position was desperate. Li Siye changed his mind and agreed with Duan, and Liang subsequently commissioned Li Siye to lead the Anxi forces to rendezvous with Emperor Suzong at Fengxiang (鳳翔, in modern Baoji, Shaanxi), with Duan as his deputy. On the way, under LI Siye's orders, the army followed strict discipline and did not cause damage to the people. When LI Siye's army arrived at Fengxiang, Suzong was very pleased, and had LI Siye serve as a commander of the imperial forces, along with Guo Ziyi and Pugu Huai'en. At this time, it was said that Li Siye's weapon of choice was still his staff, and that he would lead his forces into battle, charging while swinging his staff, and was without rival. He was made the commander of the expedition forces from Anxi and Beiting (北庭, headquartered in modern Changji Hui Autonomous Prefecture, Xinjiang).