Emperor Taizong agreed. In winter 641, indeed, with Yi'nan anticipating that Emperor Taizong would soon be offering sacrifices to heaven and earth at Mount Tai and would take his soldiers with him, he believed that he could destroy Ashina Simo quickly. He therefore had his son Dadu (大度) to head Xueyantuo troops, along with conscripted troops from vassal tribes Tongluo (同羅), Pugu (僕骨), Huige, Mohe, and Xi (霫), launch a major attack on Eastern Tujue. Ashina Simo could not resist the attack, and withdrew within the Great Wall to Shuo Prefecture (朔州, roughly modern Shuozhou, Shanxi) and sought emergency aid. Emperor Taizong sent the generals Zhang Jian, Li Shiji, Li Daliang, Zhang Shigui (張士貴), and Li Xiyu (李襲譽), to attack Xueyantuo to assist Ashina Simo, with Li Shiji in overall command. Around the new year 642, Li Shiji dealt a major defeat to Dadu, who fled after heavy casualties. Emperor Taizong, while sending an emissary to rebuke Yi'nan, did not take further actions against Xueyantuo at this point. It was said that Yi'nan continued to be unhappy with Eastern Tujue's existence, and continued to harass Eastern Tujue. (Yi'nan, however, tried to maintain peaceful relations with Tang, and at one point was engaged to marry Emperor Taizong's daughter Princess Xinxing, but Emperor Taizong regretted the marriage agreement and broke it in 643 under the pretense that Yi'nan's offer of bride price (with livestock) was not paid for on time.) When Emperor Taizong sent further emissaries to order Yi'nan from attacking Eastern Tujue, Yi'nan's response was: