time, Ashina Duojishi's brother and successor, the Chuluo Khan Ashina Qilifu planned a major assault on Tang, and as part of the plan, he requested Dou to cross the Taihang Mountains and meet him at Jin (晉州, roughly modern Linfen, Shanxi) and Jiang (絳州, roughly modern Yuncheng, Shanxi) Prefectures, but Ashina Qilifu soon died and did not carry out the campaign. At the same time, Dou also executed Song, who often gave Dou honest and valid criticism, after false accusations, and it was said that thereafter no one dared to criticise Dou further, and that Dou's governance began to suffer because of it. In spring 621, Dou defeated Meng Haigong and captured him, keeping him as a general. Defeat and death Meanwhile, around the same time, the Tang general Li Shimin (Emperor Gaozu's son) was leading a major attack on Zheng's capital Luoyang. Wang Shichong, unable to fend off the attack by himself, sought aid from Dou Jiande, even though Xia and Zheng had previously had a poor relationship. Dou's official Liu Bin () suggested that he aid Zheng, reasoning that if Tang destroyed Zheng, Xia would be under Tang's threat thereafter—and that if he were victorious, he could then consider seizing Zheng territory. Dou agreed and sent messengers to Wang, promising support, while sending his official Li Dashi to Li Shimin, requesting that he end his campaign against Luoyang. Li Shimin, however, detained Li Dashi and did not respond. Dou then merged the troops from Meng Haigong and Xu Yuanlang (whom he had defeated earlier as well) and headed from Meng's headquarters at Cao Prefecture (in modern Heze as well) toward Luoyang, joining forces with the Zheng general Guo Shiheng (), and he defeated several cities held by Tang. He notified Wang that the Xia forces were on the way, and also wrote Li Shimin to again request Li Shimin to withdraw and return the seized land to Zheng. When Li Shimin discussed this proposal with his generals, most believed that they should avoid Dou, but Guo Xiaoke () disagreed, believing that this was a good chance to destroy both Wang and Dou. He suggested for Li Shimin to proceed to Hulao Pass, east of Luoyang, and defend it against the coming Dou attack. Li Shimin agreed. He left his brother Li Yuanji and the general Qutu Tong () at Luoyang, continuing the siege, while proceeding to Hulao himself.