Prince of Jin and Zhu Quanzhong's son Zhu Zhen as the Emperor of Later Liang. In 916, Li Cunxu made his adoptive brother Li Siyuan the military governor of Anguo Circuit (安國, headquartered in modern Xingtai, Hebei). An was then serving under Li Siyuan with the officer rank of Zhongmenshi () and faithful to Li Siyuan, and Li Siyuan trusted him deeply. He was friendly with Li Siyuan's sons Li Congrong and Li Conghou from Li Congrong's and Li Conghou's childhood, and the brothers respected him. However, he did not have a good relationship with Li Siyuan's adoptive son Li Congke, as there was a time when they were drinking and got into an argument, and Li Congke beat him badly. Li Congke later apologized, but An continued to bear a grudge against Li Congke. For more than a decade, An followed Li Siyuan in his campaigns serving under Li Cunxu, who eventually declared himself emperor of a new Later Tang (as its Emperor Zhuangzong) and conquered Later Liang. However, by 926, the Later Tang realm was overrun by mutinies after Emperor Zhuangzong had, without good cause, executed two of the major generals, Guo Chongtao and Zhu Youqian. When Li Siyuan was sent to battle the mutineers at Yedu (鄴都, in modern Handan, Hebei), Li Siyuan's own forces mutinied and forced him to join the Yedu mutineers. After Li Siyuan freed himself from the Yedu mutineers, he considered returning to his own circuit Chengde Circuit (成德, headquartered in modern Shijiazhuang, Hebei), which he was then the military governor of, to consider his next step, but An and Li Siyuan's deputy Li Shaozhen pointed out that that exposed him to be defenseless against charges that another general, Li Shaorong, might lay against him, convinced him to head south back toward the capital Luoyang instead. Subsequently, with Li Shaorong indeed accusing Li Siyuan of treason and cutting off Li Siyuan's attempts to send messengers to Emperor Zhuangzong to defend himself, Li Siyuan decided to indeed rebel, and therefore had An draft summons on his behalf, asking other generals to join his army, as he advanced toward Kaifeng (Bian prefecture). Li Siyuan quickly entered Kaifeng, and then from there advanced toward Luoyang. Emperor Zhuangzong organized an army to try to resist him, but was shortly thereafter killed in a mutiny at Luoyang. When Li Siyuan arrived at Luoyang, he was welcomed into the