Soon, however, Hou, believing that Emperor Wu, who engaged in peace talks with Eastern Wei and appeared to be willing to betray him to exchange for Xiao Yuanming, rebelled in summer 548. Hou's army quickly advanced on Jiankang, assisted by Xiao Gang's cousin Xiao Zhengde the Prince of Linhe, putting Jiankang under siege. Emperor Wu put Xiao Gang in charge of the defenses, but Xiao Gang was unsuccessful in preventing the outer city from falling. The imperial troops were forced to withdraw into the palace. When Hou then claimed that it was the corrupt official Zhu Yi that he wanted to kill, Xiao Gang confirmed that Zhu was indeed corrupt, but advised against executing Zhu in that it would serve nothing in the campaign against Hou. Soon, Hou declared Xiao Zhengde emperor. While provincial troops gathered near Jiankang to try to relieve the besieged palace, those troops, commanded by Liu Zhongli () and Xiao Guan, were unsuccessful, and ultimately, after Liu was nearly killed in a battle, Liu refused to engage Hou any further, leaving the palace troops to fend for themselves. (Xiao Yi and Xiao Ji, although they each had large numbers of troops in their domains, largely stood and sent only token troops.) In winter 548, Hou's general Fan Taobang () offered to rebel against Hou, and while Emperor Wu was initially in favor of the idea, Xiao Gang spoke against it, and it was not carried out. Soon, Fan was killed by Hou, and an opportunity was lost.