The transition from Sui to Tang started roughly around 613 when Emperor Yang of Sui launched his first of three failed campaigns against Goguryeo, and ended in 628, when Emperor Gaozu’s son Li Shimin annexed the agrarian rebel ruler Liang Shidu’s state of Liang.
As of 611, Sui Dynasty had just enjoyed more than two decades of peace and prosperity, as China had been united under it since it destroyed Chen Dynasty in 589.
When Goguryeo’s king Yeong-yang refused to pay homage to Emperor Yang in 610, Emperor Yang decided to plan a campaign to conquer it, and both he and the people believed that the conquest would be easy.
The logistics of staging the attack on Goguryeo, however, took much human and other tolls, as the building of a fleet and the shipping of food and other supplies to the base of operations, Zhuo Commandery (涿郡), caused major disruptions in the farming cycle and major deaths in those conscripted to ship the supplies to Zhuo Commandery.
Emperor Yang launched his first campaign against Goguryeo in 612, crossing the Liao River into Goguryeo territory in spring 612.
Emperor Yang, however, was never able to capture Liaodong. By fall 612, Emperor Yang was forced to terminate the campaign and withdraw as well, with only minor territorial gains. About 300,000 men had been lost in the campaign.
Emperor Yang launched a second campaign against Goguryeo in 613, even though the agrarian rebellions were becoming more numerous and serious. While he was sieging Liaodong, however, the general Yang Xuangan, in charge of logistics near the Sui eastern capital Luoyang, rose in rebellion, attacking Luoyang.
Emperor Yang launched a third campaign against Goguryeo in 614. Goguryeo submitted, sending Yang Xuangan’s confederate Husi Zheng (斛斯政) back to Sui as a sign of submission. Emperor Yang terminated the campaign, but when he again summoned Gao Yuan to pay homage to him, Gao Yuan ignored his summons.
Emperor Yang began to plan a fourth campaign, which, however, he was never able to launch.
Meanwhile, in the fall of 615, while Emperor Yang and Empress Xiao were conducting a tour of the northern frontier, Qimin Khan’s son and successor Shibi Khan launched a surprise attack against Yanmen Commandery as a reprisal for the emperor’s proposal to give a princess to his brother and for the treacherous murder of one of his close advisors.
Emperor Yang followed the advice of Su Wei and Yuwen Shu and reneged on most of his promises, causing great resentment among the military.
Because of increasing agrarian rebel activities in northern China, however, Emperor Yang did not return to Chang’an or stay at Luoyang, but went to Jiangdu (江都) in fall 616. With his departure from Luoyang, the rebels near Luoyang coalesced under the leadership of Yang Xuan’gan’s former strategist Li Mi, who was proclaimed the Duke of Wei and considered the presumptive eventual emperor by most rebel leaders throughout northern China. Li, however, was not able to capture Luoyang and never claimed the imperial title.
Meanwhile, Yang 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 Yang 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.
By 617, a number of other major rebel leaders also began to control significant portions of territory.
Several of these rebel leaders—including Li Yuan, Liu Wuzhou, Liang Shidu, Dou Jiande, and Gao Kaidao—formally submitted to Ashina Duojishi and received Eastern Tujue military aid, with Ashina Duojishi’s strategy apparently to keep China divided. In winter 617, Li Yuan captured Chang’an, declaring Yang You emperor, while honoring Emperor Yang as Taishang Huang (retired emperor); these declarations were not recognized by most of Sui territory, which still recognized Emperor Yang as emperor. Li himself became regent with the title of Prince of Tang.
Emperor Yang, while realizing that the empire was in turmoil, felt secure under the protection of the elite Xiaoguo Army (驍果) at Jiangdu, and while he sent his general Wang Shichong to Luoyang to try to defend Luoyang against Li Mi’s attacks, appeared to do little to quell the rebellions otherwise.
Soon, news of Emperor Yang’s death spread throughout the empire. At Chang’an, Li Yuan responded by having Yang You yield the throne to him, establishing the Tang Dynasty as Emperor Gaozu.
At Luoyang, seven of the leading officials declared another grandson of Emperor Yang, Yang Tong the Prince of Yue, emperor, and Yang Tong was recognized as Sui’s emperor by most of the commanderies that still recognized Sui sovereignty. With both the Sui administration at Luoyang and Li Mi fearing Yuwen’s northward advancement, they formed a temporary alliance in which Li Mi recognized Yang Tong as his sovereign. Li Mi was later killed by Tang forces when he tried to reestablish his own independence.
After Xue Ju died in early 618 and was succeeded by his son Xue Rengao, the Tang general Li Shimin the Prince of Qin (Emperor Gaozu’s son) defeated and killed Xue Rengao, annexing his Qin state into Tang.
In late 619, Tang forces, commanded by Li Shimin, began a counterattack against Liu Wuzhou. By summer 620, Li Shimin had defeated Liu, who abandoned his territory and fled to Eastern Tujue. His Dingyang state was integrated into Tang.
In spring 621, with Dou approaching, Li Shimin advanced east to the important Hulao Pass and held position there. When Dou engaged him, he defeated Dou and captured him.
Also in 621, Emperor Gaozu’s nephew Li Xiaogong the Prince of Zhao Commandery attacked Xiao Xian’s Liang state, putting the Liang capital Jiangling under siege.
In spring 622, Li Shimin defeated Liu Heita, forcing Liu to flee to Eastern Tujue, but Liu returned later that year with Eastern Tujue aid, reoccupying the former Xia territory. In winter 622, Li Shimin’s older brother Li Jiancheng the Crown Prince defeated Liu again.
In fall 623, however, with Li Fuwei at Chang’an, Li Fuwei’s lieutenant Fu Gongshi rebelled at Danyang, declaring himself the Emperor of Song and controlling the territory formerly under Li Fuwei’s control. By 624, Li Xiaogong had defeated and killed Fu, reintegrating Song territory into Tang, while Gao, faced with a coup led by his subordinate Zhang Jinshu (張金樹), committed suicide, and his Yan state was also integrated into Tang.
By 628, with Eastern Tujue in internal turmoil due to disagreements between the Jiali Khan Ashina Duobi (Ashina Duojishi’s younger brother) and the subordinate Tuli Khan, Ashina Shibobi (阿史那什鉢苾, Ashina Duojishi’s son), it was no longer able to protect Liang Shidu, and under Tang siege, Liang Shidu’s cousin Liang Luoren (梁洛仁) killed Liang Shidu and surrendered. All of China was now under the rule of Emperor Taizong.
Edited by staff