Meanwhile, in the fall of 615, while Emperor Yang and Empress Xiao were conducting a tour of the northern frontier, Qimin Khan's son and successor Shibi Khan launched a surprise attack against Yanmen Commandery as a reprisal for the emperor's proposal to give a princess to his brother and for the treacherous murder of one of his close advisors. The khan's wife Princess Yicheng, a relative of the emperor who had been personally honored by the empress during an earlier visit, sent them a secret warning about the Turkic attack. The imperial entourage fled to the fortified commandery seat at present-day Daixian, Shanxi, where Shibi Khan's forces besieged them on September 11. Yu Shiji advised the emperor to raise morale among the soldiers by promising promotion and rewards, as well as promising to abandon the unpopular campaigns against Goguryeo; this had the desired effect and Sui forces loyally rushed to the area to lift the siege. Meanwhile, however, the empress's brother Xiao Yu had gotten the emperor to seek further assistance from Princess Yicheng, who was administering military affairs at home in her husband's absence. She sent a false report to the khan that a northern attack had been launched against the khaganate; between this report and others announcing the Chinese reinforcements, Shibi Khan lifted the siege and returned north. With credit for his salvation muddled, Emperor Yang followed the advice of Su Wei and Yuwen Shu and reneged on most of his promises, causing great resentment among the military.