Battle of Hulao
The Battle of Hulao, or Battle of Sishui, was a decisive Tang victory over the rival Zheng and Hebei-based Xia polities during the transition from Sui to Tang. The battle took place during the Luoyang–Hulao campaign on 28 May 621 when a Xia army – led by Dou Jiande, ruler of Xia – was defeated attacking a smaller Tang army – led by Prince Li Shimin – entrenched at the strategic Hulao Pass.
Li launched the Luoyang–Hulao campaign in August 620, attacking eastwards and quickly besieging Wang Shichong, ruler of Zheng, in Luoyang. Zheng attempts to break the siege failed, and they appealed to Xia for help. In April 621, Dou led a Xia army of 100,000–120,000 troops westward to break the siege. Instead of retreating back to the Tang heartland in Shanxi, Li maintained the siege and took a small part of the Tang army further east to the Hulao Pass to block the Xia. A stalemate ensued for a few weeks; the Tang refused to fight outside of their positions, and the Xia refused to outflank Li or redirect their offensive to Shanxi.
Li broke the stalemate at the end of May by feinting the departure of a large part of the Tang cavalry. On the morning of 28 May, the Xia issued a challenge by arraying for battle in the open. The Tang sapped the endurance and unit cohesion of the waiting Xia by delaying the main action. A Tang cavalry probe at noon developed into a general attack as Xia cohesion collapsed. The Xia retreat, disrupted by poor communications and Tang cavalry in the rear and flanks, turned into a rout. The Xia defeat and Dou's capture led to the surrender of both Zheng and Xia to Tang. Tang emerged with control over the central plains, a dominant position from which it completed conquering China in 628.
Background
During the later reign of the second emperor of the Sui dynasty, Yang, 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 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 north China 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' and 'King'. Among the most well-positioned contenders was Li Yuan, Duke of Tang and governor of Taiyuan in the northwest. 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 imperial capitals of Daxingcheng 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.
Li Shimin besieges Wang Shichong at Luoyang
Fresh from his crushing victory over Liu Wuzhou, in August 620 Li Shimin, with an army of 50,000 men, began his advance from Shanxi towards Luoyang. The strategic aim of the Tang prince was to capture the Yellow River valley up to the sea, thus separating the territories held by rival regimes in the north from any allies in the south, particularly after Du Fuwei, a rebel leader who controlled the Huai River area, chose to acknowledge Tang authority.Starting from Shenzhou, Li Shimin's progress was swift, advancing against little resistance as Wang feared risking an open confrontation and remained behind the walls of Luoyang. By September, Tang troops had begun to establish a ring of fortified camps around the city. Wang's offers of a settlement based on a partition of the empire were rejected by Li Shimin. While both sides skirmished around Luoyang, each trying to protect or prevent the supply convoys coming into the city, Tang detachments had penetrated further south, east and north, triggering the defection of most of central Henan from Wang's control. By the end of the year, only the distant cities of Xiangyang and Xuzhou remained under Wang's control, but were unable to provide any assistance. The monks of the nearby Shaolin Monastery also sided with Li Shimin, defeating a detachment of Wang's army at Mount Huanyuan and capturing his nephew, Wang Renze.
Isolated in his capital and the territory immediately around it, Wang Shichong was growing desperate and more aggressive, launching two major attempts to break out of the Tang blockade in early 621. Both battles were hard fought, but eventually won by the Tang, largely thanks to the intervention of Li Shimin with his bodyguard of 1,000 heavily armed horsemen. The failure of these attempts meant that the siege became ever closer, with siege engines employed to support daily attacks on the city from all sides. The supply situation in Luoyang grew steadily worse as the siege continued into winter and then spring. By March, people were reportedly shifting through dirt to find traces of food, or ate cakes of rice and mud. No one was spared from the suffering, not even the highest officials; and of the 30,000 prisoners held by Wang in his palace, barely a tenth were left alive. Nevertheless, Wang refused any suggestion of surrender, placing his final hopes on an intervention by Dou Jiande, to whom he had sent envoys already in late 620.
Dou Jiande marches west and Li Shimin occupies Hulao
During the siege of Luoyang, Dou Jiande and the Tang had been engaged in negotiations, but these had been inconclusive: Dou could not ignore the threat posed by the Tang, but was as yet unwilling to effect a complete breach, and made some conciliatory gestures such as releasing a Tang princess he had captured in 619. When Wang's pleas arrived at his court, Dou was persuaded by his councillor Liu Bin that the situation presented both danger and opportunity: if Luoyang fell, the Tang would next turn against Dou, but if Dou intervened and saved Luoyang, it would be easy to oust the weakened Wang and annex Henan to his own Xia state. It was therefore probably by design that Dou waited until April, when Wang's situation had become critical, before he began marching west to relieve the siege of Luoyang.The Tang launched an attack from their bases in Shanxi against Dou's flank, hoping to divert his attention, but in vain. Dou had enough men to strongly garrison his territory, while still mustering a huge force for marching against Li Shimin. The 10th-century Old Book of Tang and the 11th-century Zizhi Tongjian put Dou's army at 100,000 strong, while the 8th-century works Tongdian and Taizong zun shi, raise it to 120,000 men. Although possibly exaggerated, an army of this size was well within the capabilities of the time. The Xia army was accompanied by a similarly large supply train, comprising both carts and boats.
The approach of the Xia army placed the Tang army at Luoyang in a predicament: with no prospects of reinforcements, and with the loyalty of the recently captured cities in Henan suspect, remaining in place to be caught between Wang's men in Luoyang and Dou's army was a recipe for disaster. The older, more experienced and cautious of Li Shimin's generals suggested that he abandon the siege and retire west to Guanzhong, but the Tang prince refused to heed them, as this would mean abandoning the entirety of eastern China to Dou Jiande. Leaving control of the populous northeastern plain to Dou would strengthen his regime, and allow him to expand south to the south, where Du Fuwei and other Tang clients would be forced to submit. Not only would this mean abandoning the unification of the empire, but it would place the Tang regime itself in peril. In what his modern biographer C. P. Fitzgerald called "the most critical military decision of his life", Li Shimin opted to confront the Xia army with a part of his forces, while leaving most of his army to maintain the siege of Luoyang. This was a risky gamble, as a defeat would risk eliminating the main Tang army, and opening the path for Dou to capture not only Luoyang, but Shanxi and Chang'an itself.
Leaving the siege of Luoyang in the hands of his younger brother Li Yuanji and the general Qutu Tong, Li Shimin took 3,500 men to the Hulao Pass, some to the east of Luoyang, which he occupied on 22 April. His force was augmented by the garrison of the local town, but is unlikely to have exceeded 10,000 men, albeit representing some of the best troops in the Tang army. The Hulao Pass was formed by the ravine of the Sishui river. Lined on both banks by escarpments and steep hills, rising in the south to the Song mountains, it possessed major strategic importance, as the east–west road along the Yellow River's south bank crossed it. Fitzgerald, who visited the area himself in the early 20th century, described this "Chinese Thermopylae" as follows: