After the defeat of his ally Wu Yuanji in 817, he submitted to imperial authority, and, after his death, his brother Wang Chengyuan left the circuit, ending his family's hold on Chengde.
Meanwhile, Emperor Xianzong considered forcibly imposing another military governor on Chengde and attacking it if Wang Chengzong refused to yield.
The chancellor Pei Ji and the imperial scholar Li Jiang believed such a course of action to be imprudent, pointing out that Chengde would be difficult to conquer.
The powerful eunuch Tutu Chengcui, however, advocated a campaign against Chengde, as did Lu Congshi (盧從史) the military governor of Zhaoyi Circuit (昭義, headquartered in modern Changzhi, Shanxi).
When Emperor Xianzong sent the official Pei Wu (裴武) to Chengde to comfort the soldiers, Wang offered to surrender De and Di Prefectures.
After Pei returned to the Tang capital Chang'an, Emperor Xianzong issued an edict naming Wang military governor of Chengde, while forming a separate circuit, Baoxin Circuit (保信), out of De and Di Prefectures and naming a relative of Wang's by marriage, Xue Changchao (薛昌朝) the prefect of De Prefecture, its military governor.
Liu Ji the military governor of Lulong Circuit (盧龍, headquartered in modern Beijing) also attacked Chengde on his own.
Wang's ally Li Shidao the military governor of Pinglu Circuit (平盧, headquartered in modern Tai'an, Shandong) also made the same request on his behalf.
[3] Meanwhile, by 815, Emperor Xianzong's forces were deeply entrenched in a campaign against another ally of Wang Chengzong's, Wu Yuanji, who ruled Zhangyi Circuit (彰義, headquartered in modern Zhumadian, Henan).
In response, Wang pillaged the territories of Lulong, Henghai (橫海, headquartered in modern Cangzhou, Hebei), and Yiwu.
[5] By 817, the expenditures of the imperial forces in waging campaigns against Chengde and Zhangyi simultaneously were so high and the returns so low that Emperor Xianzong was reconsidering his strategy.
[5] In winter 817, imperial forces under Li Su entered Zhangyi's capital Cai Prefecture (蔡州) and captured Wu Yuanji, whom Emperor Xianzong subsequently executed.