In response, in 611, in northern China, those who refused to be conscripted began to revolt as agrarian rebels, led by leaders such as Wang Bo (王薄) and Liu Badao (劉霸道).

Emperor Yang personally led part of the army to put the important city Liaodong (遼東, in modern Liaoyang, Liaoning) under siege, while he sent the Xianbei generals Yuwen Shu and Yu Zhongwen (于仲文) to lead the rest of the army deep into Goguryeo territory, heading toward the Goguryeo capital Pyongyang, joined by the fleet commanded by the southern Chinese general Lai Hu'er.

Emperor Yang, however, was never able to capture Liaodong, while Yuwen and Yu, advancing nearly to Pyongyang, were defeated by the Goguryeo general Eulji Mundeok and forced to withdraw with heavy losses.

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,[2] where Shibi Khan's forces besieged them on September 11.

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.

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.

[2] Despite (or perhaps because of) increasing agrarian rebel activities in northern China, Emperor Yang did not return to Chang'an or stay at Luoyang, but went to Jiangdu (江都, in modern Yangzhou, Jiangsu) in fall 616.

Meanwhile, Yuchi Yichen made an attempt to destroy the rebels north of the Yellow River, but while he enjoyed some successes, Emperor Yang and his prime minister Yu Shiji, fearing Yuchi Yichen's military strengths, recalled him under guise of a promotion, allowing the rebel activities north of the Yellow River to reinvigorate themselves and become difficult to control, under the leadership of Dou Jiande.

Meanwhile, Emperor Yang, while realizing that the empire was in turmoil, felt secure under the protection of the elite Xiaoguo Army (驍果) at Jiangdu.

Not wanting to return to north China, he considered officially moving the capital to Danyang (丹楊, in modern Nanjing, Jiangsu, south of the Yangtze River).

Meanwhile, the Xiaoguo Army soldiers, who were predominantly northerners and missed their homes, began to desert, and this met with heavy-handed punishment from Emperor Yang.

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

Around the same time, the lower Yangtze region, which had been in a state of confusion ever since Emperor Yang's death, was coalescing around three different competing figures—the former Sui official Shen Faxing, who declared himself the Prince of Liang and controlled much of the territory south of the Yangtze; the rebel leader Li Zitong, who controlled Jiangdu and the surrounding regions, declaring himself the Emperor of Wu; and Du Fuwei, who submitted to Tang and was created the Prince of Wu.

Many Zheng cities surrendered to Tang, forcing Wang Shichong to seek aid from Dou Jiande's Xia state.

Dou, reasoning that if Tang destroyed Zheng, his own Xia would be cornered, agreed, and he advanced south toward Luoyang, seeking to lift the siege.

Wang's Zheng state and Dou's Xia state were annexed by Tang, although former Xia territory soon rose under the leadership of Dou's general Liu Heita, who declared himself the Prince of Handong, and modern Shandong, which had been controlled by the agrarian leader Xu Yuanlang but had successively submitted to Zheng and then to Tang, rose as well under Xu, who declared himself the Prince of Lu.

By this point, other than Liang Shidu and Gao Kaidao in the extreme north, China was largely reunited, if somewhat nominally, under Tang rule.