The Battle of Hulao (Chinese: 虎牢之戰) or Battle of Sishui (汜水之戰, Wade–Giles: Ssŭ Shui), on 28 May 621 was the main and final battle of the Luoyang–Hulao campaign between the rival Tang, Zheng, and Xia regimes during the transition from Sui to Tang. It was a decisive victory for the Tang prince Li Shimin, through which he was able to subdue two rival warlords, Dou Jiande who headed the Xia regime in Hebei, and Wang Shichong, the self-declared emperor of the Zheng dynasty. The battle was fought at the strategically important Hulao Pass, east of Luoyang.
Following victories in the west that had established his credentials as a general, in August 620 Li Shimin marched against Wang Shichong. Tang troops blockaded Wang in his capital of Luoyang, while seizing the rest of Henan province. After failing in his efforts to break through the Tang siege, and suffering from ever greater privations, Wang solicited help from Dou Jiande. In April 621, Dou Jiande led a 100,000–120,000 strong army west to confront the Tang. Li Shimin’s generals urged him to retreat west and protect the Tang core territory at Shanxi, but doing so would surrender the northeastern plains, at that time the heartland of China, to Dou. Consequently, Li Shimin took a gamble by leading a small force east to occupy the strategic Hulao Pass, while the bulk of his army was left behind continuing the siege of Luoyang. Ensconced in favourable defensive positions, the Tang managed to hold up the Xia advance. A large and heterogeneous army, the Xia lacked the flexibility to either outflank Li Shimin’s position or abandon the Luoyang campaign and attack the exposed Tang heartland at Shanxi. As a result, the standoff between the two armies continued for several weeks.
Finally, when he judged the situation ripe, Li Shimin feigned detaching part of his forces north to entice an attack. When Dou took the bait and advanced to the Tang positions in battle order, Li Shimin held back his own troops for several hours, until the troops of Dou Jiande were exhausted from being made to wait in formation under the sun for the entire morning. Once signs of disorder began appearing among them, Li Shimin attacked, breaking the opposing army and capturing Dou Jiande. Subsequently, Wang Shichong, left with no other choice, surrendered Luoyang. Both his and Dou Jiande’s states were absorbed by the Tang. Dou Jiande was later executed, resulting in some of his followers, led by Liu Heita, raising an unsuccessful rebellion against the Tang. Hulao marked the decisive turning point in the civil wars that followed the collapse of the Sui Dynasty, after which the eventual victory of the Tang was never in doubt.
During the later reign of the second emperor of the Sui Dynasty, Yang (r. 604–618), the dynasty’s authority began to wane: the immense material and human cost of the protracted and fruitless attempts to conquer the Korean kingdom of Goguryeo, coupled with natural disasters, caused unrest in the provinces, and military failures eroded the emperor’s prestige and legitimacy (“Mandate of Heaven“) among the provincial governors. Yang nevertheless continued to be fixated on the Korean campaigns, and by the time he realized the gravity of the situation, it was too late: as revolts spread, in 616, he abandoned the north and withdrew to Jiangdu, where he remained until his assassination in 618.
Local governors and magnates rose to claim power in the wake of Yang’s withdrawal. Nine major contenders emerged, some claiming the imperial title, others, contenting themselves, for the time being, with the more modest titles of “Duke” (gōng) and “King” (wáng). Among the most well-positioned contenders was Li Yuan, Duke of Tang and governor of Taiyuan in the northwest (modern Shanxi). A scion of a noble family related to the Sui dynasty, and with a distinguished career behind him, Li Yuan was an obvious candidate for the throne. His province possessed excellent natural defences, a heavily militarized population and was located near the capitals of Daxingcheng (Chang’an) and Luoyang. In autumn 617 Li Yuan and his sons, Li Shimin and Li Jiancheng, led their troops south. In a lightning campaign they defeated the Sui forces that tried to bar their way and, on 9 November, Li Yuan’s troops stormed Chang’an. Li Yuan was now firmly placed as a major contender for the imperial throne, and on 16 June 618 he proclaimed himself the first emperor of the Tang Dynasty.
In a series of campaigns in 618–620 the Tang, led by the talented Li Shimin, managed to eliminate their rivals in the northwest and repel an attack by Liu Wuzhou, who had taken control of Shanxi, but they still had to expand their control to the northeastern plain and the modern provinces of Hebei and Henan, which, in the words of historian Howard J. Wechsler, would decide whether the new dynasty “would remain a regional regime or whether they would succeed in uniting the country under its control”. By early 620, two major regimes had established themselves over this region. Henan was controlled by the Luoyang-based Wang Shichong, a former Sui general who declared himself the first emperor of the Zheng dynasty after defeating another rebel leader, Li Mi, at the Battle of Yanshi and absorbing his army and territories. Hebei was ruled by the one-time bandit leader Dou Jiande, who had risen in revolt against the Sui already in 611. From his base at Mingzhou in south-central Hebei he had expanded his control south towards the Yellow River, claiming the title of “King of Xia”. Like Wang and the Tang, he too made use of the pre-existing Sui officialdom and administrative apparatus to maintain his realm.
In 619, Dou defeated the Tang army under Li Yuan’s cousin Li Shentong and captured their territories north of the Yellow River, while from Luoyang Wang was a constant threat to the cities of the lower Yellow River that had only recently acknowledged Tang authority. The two men are presented as diametrically different characters in the sources: while Dou was chivalrous and successfully extended his territories by judicious moderation, Wang’s arbitrariness and lack of courtesy quickly alienated many of his own supporters, leading two of his most distinguished generals, Qin Shubao and Luo Shixin, to desert him and join the Tang. The Tang began launching raids against Wang, causing morale to drop and many of his men to defect. Wang was forced to take hostages from the families of his own generals to ensure their loyalty, and impose kin punishment for any trespassing. Although up to 30,000 people ended up as virtual prisoners in his palace city in Luoyang, these acts only served to further undermine his regime.