In 621, when the Tang dynasty general Li Shimin (later Emperor Taizong) attacked Wang Shichong the Emperor of Zheng, who ruled the modern Henan region, Dou believed that if Tang were able to destroy Zheng, his own Xia state would suffer the same fate, and therefore went to Wang's aid, against the advice of his strategist Ling Jing (凌敬) and his wife Empress Cao.

One of the men from Dou's home county, Sun Anzu (孫安祖), whose house had recently been destroyed in the flood and whose wife had starved to death, was also conscripted.

How can you stay here and be a fleeing felon?He therefore gathered several hundred men of the region and gave them to Sun to lead, to become bandits at the nearby Gaoji Pond (高雞泊).

Meanwhile, there were other bands of bandits in the same commandery, Qinghe (清河, roughly modern Xingtai, Hebei), led by Zhang Jincheng (張金稱) and Gao Shida (高士達).

In 616, Guo Xuan (郭絢) the governor of Zhuo Commandery (涿郡, roughly modern Beijing) led a Sui army against Gao.

If you fight him now, I am afraid that you, Duke, will not be able to defeat him.Gao disagreed, and, leaving Dou in charge of the base, engaged Yang.

In spring 617, Dou claimed the title of Prince of Changle and also changed the era name that his followers used, from Emperor Yang's Daye (大業) to Dingchou[4] (丁丑), thus subtly signifying an official break from Sui.

Dou instead planned a surprise attack, leading 280 of his best soldiers personally as the forward assault force at night while having the rest of his men trail.

He made an agreement with them—that if they reached Xue's camp at night, they would make a surprise attack, but if it was already the day by the time that they arrived, they would surrender.

In spring 618, after Li had a major victory over the Sui general Wang Shichong, whom Emperor Yang had sent from Jiangdu (江都, in modern Yangzhou, Jiangsu) to aid the eastern capital Luoyang, Dou, along with several other rebel leaders, sent a submission to Li, then carrying the self-declared title of Duke of Wei, urging him to take imperial title.

Dou wrote back in humble and polite terms, but declined with the excuse that he needed to defend an attack from Luo Yi (a former Sui official who had seized Zhuo Commandery himself) from the north.

Soon, news arrived from Jiangdu that Emperor Yang had been killed in a coup led by the general Yuwen Huaji.

Further, at the suggestion of his officials Song Zhengben (宋正本) and Kong Deshao (孔德紹), he changed his title to Prince of Xia.

Dou captured Yuwen and formally greeted Emperor Yang's wife Empress Xiao, referring to himself as "your subject."

In summer 619, Wang Shichong had Yang Tong yield the throne to him, ending Sui and establishing a new state of Zheng.

In response, Dou cut off relations with Wang, and began to take on imperial style in his edicts and ceremonies, although he was still using the title of Prince of Xia and not emperor.

Meanwhile, with certain prefectures north of the Yellow River having submitted to either Tang or Zheng, he carried out a campaign to capture them over the next several months, and was largely successful.

Around the new year 620, Li Shiji further suggested to Dou that they should attack Cao (曹州) and Dai (戴州) Prefectures (together making up about modern Heze, Shandong), then controlled by the agrarian leader Meng Haigong (孟海公) and nominally submitting to Zheng.

It was described that by this point, Dou was encouraging his people to farm, and that his realm was peaceful, without banditry, and that merchants and travelers were comfortable enough to spend the night in the wilderness.

Dou agreed to the peace proposal, and sent Princess Tong'an back to Tang, although he continued to hold Li Shentong.

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 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 (郭士衡), and he defeated several cities held by Tang.

He suggested for Li Shimin to proceed to Hulao Pass,[6] east of Luoyang, and defend it against the coming Dou attack.

He left his brother Li Yuanji and the general Qutu Tong (屈突通) at Luoyang, continuing the siege, while proceeding to Hulao himself.

Dou was wounded by a spear, but still fled, but when he reached the Yellow River, intending to cross, he suddenly fell off his horse.

The Tang generals Bai Shirang (白士讓) and Yang Wuwei (楊武威), who were chasing him, captured him and took him back to Li Shimin.

Li Shimin took Dou and Wang Shichong back to the capital Chang'an to present to his father Emperor Gaozu.

However, later on Song Zhengben and Wang Fubao were executed for false accusations, and the grand strategy submitted by Ling Jing and Lady Cao were not accepted.

That is because Heaven already showed its favor elsewhere, but also because his own strategies were not perfect.Dou Jiande is portrayed by Hong Kong actor Bobby Tsang in TVB's 1987 series The Grand Canal (大運河)