Battle of Hulao

The Battle of Hulao (虎牢之戰) on 28 May 621 was a decisive victory for the Tang Dynasty prince Li Shimin, through which he was able to subdue two rival warlords, Dou Jiande and Wang Shichong. The battle was fought at the strategically important Hulao Pass, east of Luoyang. Li Shimin led a siege on the city of Luoyang, seat of the self-declared emperor Wang Shichong, who solicited help from Dou Jiande from the east. However, Dou Jiande delayed and when he finally arrived, Li Shimin brought forces to meet the new threat. Li Shimin's army avoided conflict until the troops of Dou Jiande were exhausted and then led a cavalry charge, breaking the opposing army and capturing Dou Jiande. Afterwards, Wang Shichong, seeing no other choice, surrendered Luoyang, and 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.

Background
During the later reign of the second emperor of the Sui Dynasty, Yang (reigned 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, while the successive 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 for themselves, others contenting themselves, for the time being, with more modest titles. 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 lightning campaign they defeated the Sui forces that tried to bar their way and, on 9 November, Li Yuan's troops stormed the capital. Li Yuan was now firmly placed as a major contender for the empire, 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, soon managed to eliminate their rivals in the northwest and repulse 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 H. 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 already in 611. From his base in central Hebei he had expanded his control south to the Yellow River, claiming the title of "King of Xia". Like Wang and the Tang, he too made use of the Sui officialdom and administrative apparatus to maintain his realm.

Li Shimin's campaign against Wang and Dou Jiande's march west
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. His progress was swift: by September Tang troops had begun to establish a ring of fortified camps around the city, while detachments had penetrated further south, east and north, triggering the defection of most of central Henan from Wang's control. 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. Wang Shichong was now isolated in his capital and the territory immediately around it. The Tang were able to defeat his repeated attempts to break through the siege, and 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.

By this time, Wang's only hope for rescue was an intervention by Dou Jiande, to whom he had sent envoys already in late 620. For Dou, the situation, as presented by his councillor Liu Bin, offered 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 was critical, before he began marching west to relieve the siege of Luoyang. Dou's army was a huge force, reportedly over 100,000 strong, and the two earliest sources report 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.

At the news of his approach, some of Li Shimin's generals suggested that he abandon the siege and retire west to Guanzhong, but the Tang prince refused to heed them. He left most of his army to maintain the siege, and with the rest he marched to the Hulao Pass, some 60 mi to the east, which he occupied on 22 April. 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 Songshang mountains, it possessed major strategic importance—it has been described as a "Chinese Thermopylae"—as the east–west road along the Yellow River's south bank crossed it.

Standoff at Hulao and Dou's strategic dilemma
When Dou's army arrived before the pass, it found the walled town and the western heights behind strongly held by the Tang. Dou encamped his forces at Banzhu, a plain 10 mi east of the pass, and over the next weeks, repeatedly marched to Hulao and offered battle. Li Shimin, however, was content to remain in his powerful defensive position from which his numerically inferior force could easily hold the Xia at bay. The delay worked in his favour, as the continuing standoff only brought the garrison of Luoyang closer to starvation and surrender, and when this happened he would be able to launch his strike with the entire strength of the Tang army.

Other passes were available through the hills near Hulao, but they were smaller and equally defensible; given the size of the ZIa army, the only other alternatives would have been to bypass the Tang position entirely, either by crossing the Yellow River to the north or by venturing further south to the Huanyuan Pass. Indeed, one of Dou's civil officials, Ling Jing, suggested an alternative approach, namely to avoid any engagement with Li Shimin, cross to the northern bank of the Yellow River and strike at the Tang heartland in Shanxi, thereby both weakening the Tang and forcing them to abandon the siege of Luoyang without the Xia incurring any casualties. The plan was supported by Dou's wife, but was not adopted due to the vehement opposition of the Xia generals, who dismissed Ling Jing as an armchair general. Some sources also report that a number of the Xia generals had been bribed by Wang Shichong's agents.

Whatever the true events, military historian D. Graff opines that logistical concerns played the major role in Dou's decision to stay at Banzhu, as his huge army was utterly dependent on proximity to the Yellow River and its canal network for its supplies. In addition, the heterogeneous nature of the Xia army, containing as it did the forces of various rebel leaders Dou had defeated over the past few years, and whose loyalty was doubtful, prevented Dou from dividing his army and sending various detachments on independent missions.

Battle of Hulao Pass
In the event, after a month had passed, the Tang prince decided to force a confrontation. Li Shimin's reasons for this move are unknown; D. Graff suggests that it is "possible that he believed the morale of Dou's men had deteriorated, and it is very likely that he did not wish to allow the exposed Xia army to withdraw to safety in Hebei after Luoyang had fallen", or that he was frustrated at Luoyang's unexpectedly long resistance. At the same time, Li Shimin was evidently determined to exploit the opportunity offered by the tactical situation to score a crushing victory against Dou, which would result in the rapid absorption of his domains by the Tang.

To entice his enemy to accept battle, Li Shimin sent his cavalry to raid Dou's supply lines, and positioned his troops at Hulao so as to suggest that only a small force was present. Dou took the bait, and on the early morning hours of 28 May led a large part of his army against Hulao, deploying his troops for battle along the eastern shore of the Sishui river. Per Li Shimin's plan, the Tang troops did not come forth to deploy for battle; instead they remained in their strong defensive positions, waiting for the Xia army to tire and begin its withdrawal. Then the Tang, according to D. Graff, "would rush out and fall upon the by now demoralized and disorganized Xia army". This conformed to Li Shimin's usual blueprint, which he had already employed to prevail over Xue Rengao and Liu Wuzhou: the Tang prince let the enemy advance, stretching their supply lines, and chose a suitable, highly defensible position where to confront them; he avoided a direct confrontation, instead launching raids on his opponent's supply lines, awaiting either signs of weakness or the beginning of a retreat; he then launched an all-out attack aiming at a crushing battlefield success, which he rendered decisive by following it up with a "relentless cavalry pursuit", in D. Graff's words, to exploit it and bring about the collapse of his opponent's entire regime.

Li Shimin's plan was successful: apart from skirmishes between the two armies' cavalry, the two armies maintained their standoff from about 08:00 until noon, when the Xia troops began to show signs of thirst and weariness. Li Shimin, from a high vantage point, saw this and sent 300 cavalry in a probing attack. When he saw that the demoralized Xia recoiled from this assault, he sent more of his cavalry to turn Dou's left flank from the south. Dou reacted by ordering the withdrawal of his entire army from the river to the better defensive position offered by the eastern escarpment of the Sishui valley, but this manoeuvre created confusion in the Xia lines, breaking their battle order. Awaiting this opportunity, Li Shimin ordered his army to launch a general attack against the withdrawing Xia, himself spearheading the attack at the head of his remaining cavalry. Li Shimin always led from the front—accompanied by an elite force of 1000 black-clad, black-armoured horsemen—as indeed did most of the contemporary Chinese military leaders, who were expected to prove their personal bravery on the battlefield and motivate their men by their example, rather than stay in the rear and co-ordinate their army.

The ensuing battle was bloody, but was decided when Li Shimin and a part of his cavalry broke through the Xia lines and reached the eastern escarpment, planting the Tang banners in full view of both armies. Possibly coupled with the arrival of the flanking Tang cavalry, this development caused the complete collapse of the Xia army: trapped between the Tang forces and the eastern cliffs, 3,000 Xia soldiers fell in the field or the subsequent pursuit, but more than 50,000 were taken prisoner. These included Dou Jiande himself, who was wounded and captured while trying to find a way to cross the Yellow River.

Aftermath
The Tang victory at Hulao spelled the end for Luoyang too: bereft of any hope of rescue, Wang Shichong surrendered on 4 June, after Li Shimin displayed the captured Dou Jiande and his generals before the city walls. In stark contrast to the leniency with which the Tang treated most of their defeated rivals, Dou Jiande and Wang Shichong were soon eliminated: Dou was sent to Chang'an, where he was executed, while Wang was ostensibly allowed to retire in exile, but was killed on his way there. Dou's wife and senior officials managed to escape the Xia camp and reach the safety of Hebei, but although some wanted to continue fighting under Dou's adopted son, most, including the influentian Qi Shanxinng, regarded the outcome of the battle as a sign that the Tang possessed the "Mandate of Heaven", the divine right to rule. On 10 June, the Xia formally surrendered to the Tang, with Dou's ally Xu Yuanlang and Wang Shichong's brother Shibian following suit over the next days.

As D. Graff writes, "the great victory at Hulao was the single most decisive engagement of the civil wars that separated the Sui collapse from the consolidation of Tang authority". By defeating Dou Jiande and Wang Shichong, the Tang eliminated their two strongest rivals and brought the vital north-eastern plain under their control, thereby securing an unchallenged ascendancy over all other competing factions. Tang authority had not yet encompassed all of China, and new rebellions continued to occur for a few more years. The most notable of these occurred in late 621, when the former Xia officials in Hebei rose up in reaction to the execution of Dou Jiande, under the leadership of Dou's cavalry commander Liu Heita. Nevertheless, the eventual outcome of the civil war had been decided at Hulao, and the various rebel leaders were overcome one by one. The last of them, Liang Shidu of Shuofang, was defeated in June 628, marking the end of the civil war.

In late 629, Li Shimin, by now Emperor of China, ordered the erection of Buddhist monasteries on the sites of seven of the battles he had fought during the civil war. In a gesture that illustrated the emperor's desire to bridge and heal the divisions of the conflict, for Hulao he chose the name "Temple of Equality in Commiseration".