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.

[3][4][5] 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.

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.

[20] 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.

[26] 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.

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.

[32] 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.

[36] 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 (surviving only in fragments), raise it to 120,000 men.

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.

[41][46] Fitzgerald, who visited the area himself in the early 20th century, described this "Chinese Thermopylae"[47] as follows: The stream flows in a flat valley about a mile broad, bordered to the west by the loess hills which end in a steep slope.

To the east the stream has in past ages scoured out a low, vertical cliff, on the top of which the great plain begins; flat, featureless, dotted at intervals with villages in groves of trees.

The road from the east to Lo Yang [Luoyang] and Shensi [Shanxi] descends into the ravine, crossing the stream at the little city of Ssŭ Shui [Sishui], before entering the hills by a narrow defile among precipices.When Dou Jiande's army arrived before Hulao, Li Shimin headed a daring raid to raise his own army's morale in the face of such a numerically superior enemy.

Given the size of the Xia army, the only alternatives for Dou 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.

[52][53] Military historian David A. 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.

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.

[57][58] To entice his enemy to accept battle, Li Shimin sent his cavalry to raid Dou's supply lines, and then led a portion of his forces, with 1,000 horses across the Yellow River, giving the appearance that he had detached them to guard against an attack in the direction of Shanxi.

[41][59] 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 in challenge to the Tang.

Per Li Shimin's plan, the Tang troops did not come forth to deploy for battle; instead they remained in their strong defensive positions in the hills, waiting for the Xia army to tire and begin its withdrawal.

[60][61] This conformed to Li Shimin's usual blueprint, which he had already employed to prevail over Liu Wuzhou and the ruler of eastern Gansu, Xue Rengao: 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 Graff's words, to exploit it and bring about the collapse of his opponent's entire regime.

[62] In order to draw the Tang out to the open field, where his superior numbers would carry the day, Dou sent 300 of his cavalry to cross the Sishui stream and provoke Li Shimin to attack.

[60][64] When Li Shimin saw that the demoralized and dispersed Xia troops were thrown into confusion from this assault and struggled to put up a cohesive defence, he sent more of his cavalry to turn Dou's left flank from the south.

[60][66] 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.

[69] 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.

[62][70] Li Shimin returned to Chang'an, which he entered at the head of a triumphal procession, wearing golden armour, followed by the two captive rivals and their courts, 25 of his own generals, and 10,000 horsemen.

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.