 In the fall of 615, Emperor Yang conducted a tour of the northern frontier, exposing himself to a surprise attack by Eastern Turkish ("Dongtujue") forces under Shibi Khan, who was angered by various Chinese attempts to weaken his government.The Turks quickly overran most of Yanmen Commandery, but the khan's Chinese wife—an imperial relative who had been married to his predecessor and who had been well treated by the Empress Xiao during an earlier visit—sent warning to the emperor and empress of the attack.The imperial entourage was able to reach the commandery seat at present-day Daixian, Shanxi, where they were besieged by the Turks on September 11.