After Tang fell in 907, Yang Xingmi's domain, then ruled by his son and successor
Yang Wo, became the state of Hongnong (later known as
Wu). In 908, his officers
Zhang Hao and
Xu Wen assassinated him, and shortly after, Xu killed Zhang and supported Yang Wo's younger brother
Yang Longyan to succeed Yang Wo as the Prince of Hongnong. Xu thereafter took effective reins of the state as its regent. At this time, Li Jianxun's father Li Decheng was serving as the prefect of Run Prefecture (潤州, in modern
Zhenjiang,
Jiangsu), and had a habit of going out of the city at night. This drew Xu's suspicion, concerned that Li Decheng might be planning to act against him, so he transferred Li Decheng to Jiang Prefecture (江州, in modern
Jiujiang,
Jiangxi), farther from the Hongnong capital Guangling. Realizing that Xu suspected him, Li Decheng tried to alleviate Xu's concerns by sending Li Jianxun to pay homage to Xu. When Xu met Li Jianxun, he was impressed and stated, "If he had a son like this, he cannot be an evil man." He then gave a daughter to Li Jianxun in marriage. Despite Li Jianxun's being honored based on his father's and his father-in-law's statuses, he himself was said to be careful and (at that time) not involved in politics, although he often met with the poorer members of the intelligentsia. Most of Xu Zhixun's staff members were demoted, but Li escaped this fate (perhaps because, as the later statement by Li's wife suggested, he was in communications with Xu Zhigao). Li served as Xu's deputy military governor. By late 936, when the plans for such a transition were deep in motion, Xu Zhigao had Li Jianxun's father Li Decheng and
Zhou Ben, the two most senior generals of the realm, go to Guangling to petition Yang Pu to pass the throne to Xu Zhigao, and then come to Jinling to petition him to accept. This led Xu's advisor
Song Qiqiu, who opposed the transition, to state to Li Jianxun, "Your honored father was a great contributor to Emperor Taizu [(i.e., Yang Xingmi)], but now his accomplishments are destroyed." (By this time, Li Jianxun carried the title of
Zhongshu Shilang (中書侍郎, deputy head of the legislative bureau of government (中書省,
Zhongshu Sheng)), which was a post generally designated for a
chancellor, although it is not completely clear whether he was in fact a chancellor, as the traditional accounts did not indicate whether he also carried the title of
Tong Zhongshu Menxia Pingzhangshi (), usually a prerequisite title for a chancellor.) Xu thereafter accepted the throne, ending Wu and starting
Southern Tang. == During Southern Tang ==