Yanmen Pass
In 936, then-emperor Li Congke (Li Siyuan's adoptive son), suspicious of Shi, issued an edict transferring Shi to Tianping Circuit (天平, headquartered in modern Tai'an, Shandong). Shi, fearful of the consequences, convened his chief staff members to discuss how to react to it. Most of them advised that he resist the order. However, it was Sang who pointed out that Li Congke, as an adoptive son of Li Siyuan's, lacked legitimacy and popular support, and that if Shi had the support of Later Tang's northern rival Khitan Empire, he had chance to overthrow Li Congke. Shi thus resolved to rebel against Later Tang. He had Sang draft for him a petition to Khitan's Emperor Taizong, offering, in exchange for military support, to submit as subject and son, as well as cede Lulong Circuit (盧龍, headquartered in modern Beijing) as well as other Later Tang territory north of the Yanmen Pass to the Khitan, despite the misgivings of the general Liu Zhiyuan. Shi's offer enticed Emperor Taizong, and he agreed to launch troops.