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According to most historical sources, the Battle of Shanhaiguan itself was a several hours event, although many more important and deadly battles and skirmishes were fought days after the showdown at Shanhaiguan. In late May, Li Zicheng realized that Wu Sangui will not submit to him, therefore he led his army toward the fortress at Shanhaiguan to eliminate Wu Sangui once and for all.

In earlier attempts to analyze the events at Shanhaiguan, the author studied and wrote heavily about Qing Dynasty records on the battle at Shanhaiguan. However, as in the view of most modern-day historians, the Qing records might be exaggerated and contain inaccuracies, especially in the numbers regarding the Qing, Ming (Wu Sangui), and Shun forces. Qing historians probably wrote inaccurate accounts of the battle in order to embellish the achievements of their Manchu ancestors or masters, therefore in this paper the author will not use the Qing accounts for analysis. Instead, the author will use Angela Hsi's "Wu San-kuei in 1644: A Reappraisal" and Frederick Wakeman's "The Great Enterprise" in this paper, because Hsi and Wakeman have the most detailed accounts that most present-day accounts on Shanhaiguan have similarity with. Therefore, this paper will compare and contrast the accounts of Wakeman and Hsi.

In the Journal of Asian Studies article "Wu San-kuei in 1644: A Reappraisal," Angela Hsi wrote that Wu Sangui was in Shanhaiguan when he received the news of the fall of the capital. Therefore, he raised a militia of about 20,000 villagers and, in addition to his regular army of about 40,000 combatants, prepared to march his force against Li Zicheng in the capital. According to the accounts of several Ming courtiers, Wu Sangui defeated Li Zicheng's force through "superior strategy and tactics" and chased the retreat Shun soldiers as far as Tongzhou (near Yongping), when he received the news that the Manchus had assaulted and seized his main base of Shanhaiguan. With the Manchus in his rear, Wu Sangui then decided to surrender to the Manchus, who granted him the title of Pingxi Wang ("The Prince Who Pacifies the West" in Chinese) for allowing them to enter China Proper unmolested. The main point of Hsi's account is that Wu Sangui defeated Li Zicheng without the Manchus' aid, since the Ming courtiers did not mention the appearance of the Manchus in the battlefield and it was reported that at the time the Manchus were in the vicinity of Shanhaiguan.

In his work "The Great Enterprise," Wakeman wrote that the first clashes between Shun and Wu Sangui first took place in early May, when Li Zicheng realized that he cannot force a submission from Wu through bribes or threat. Therefore, in early May Li Zicheng sent Tang Tong and then Bai Guangen to eliminate Wu Sangui, but both were apparently defeated by Wu Sangui. Few accounts concerning the events and dimensions of those earlier clashes, if they existed at all, were kept, and if Li Zicheng can send armies on two occasions to challenge Wu Sangui, one would suggest that Li Zicheng had a large number of men to waste against Wu Sangui's tremendous garrison. However, this can also suggest that Li Zicheng might have been at a disadvantage in May, after Shun had sustained losses from two previous defeats. In any case, it was also possible that the forces of Wu and Shun simply clashed as Shun commanders patrol in the easternmost frontiers of the Shun Empire.

After defeating Tang Tong and Bai Guangen's armies outside of Shanhaiguan, Wu Sangui led his victorious Ming army toward Yongping, but by then Li Zicheng learnt of Tang Tong and Bai Guangen's misfortunes and so in mid-May he decided to personally lead the Shun army toward Shanhaiguan. Wu Sangui was aware of Li Zicheng's approach, and in response he badgered the Manchus to quickly enter the frontiers, but negotiations between the Ming and Qing commanders were only concluded a few days before the commencement of the battle. Therefore, Dorgon ordered his men on forced marches to cover the distance from Ningyuan to Shanhaiguan in one day. It was estimated that Dorgon's force constituted the vast majority of the Manchu regular army, about 60,000 soldiers. Dorgon led the main force, with his two brothers Ajige and Dodo each leading a vanguard force of about 10,000 armored cavalry. The Manchu vanguard first met the Shun vanguard in a place called Shi Pian, along the Sha He (Sha River) northwest of the Shanhaiguan fortress. Apparently, Li Zicheng sent a detachment under Tang Tong to scout ahead of the Shun main force to probe the areas beyond the Great Wall and attack Wu Sangui from the flank and rear. However, the Manchus quickly decimated Tang Tong's forces, and afterwards they approached the Shanhaiguan fortress, whose cannons greet the northern barbarians. The first things that Dorgon did were to make Wu Sangui shave his forehead (as a sign of submission to the Manchus) (5), and then ordered the Ming soldiers to wear white clothes on their armor - so that the Manchus would distinguish them from the Shun soldiers in the confusion of the battlefield.

Before the battle, Dorgon discussed with his subordinates and Wu Sangui on how to best fight Li Zicheng. At first, Dorgon favored a strategy of fighting deep into Li's rear. Dorgon reasoned that with Li Zicheng leading the bulk of his force toward Shanhaiguan, the capital's defense must be minimal, so Dorgon proposed to send the Manchu army to capture the Chinese capital by the way of Miyun. However, this plan would mean that for a duration Wu Sangui had to face Li's full might alone. In the end, Hong Chengchou, the chief Ming turncoat in the Manchu camp, advised caution against the Shun forces, and so in the end Dorgon joined forces with Wu at Shanhaiguan.

Wu Sangui arrayed his forces outside to the west of the Shanhaiguan fortress (the south-side faces the sea, and the north-side and east-side face Mongol and Manchu territories), and he gave battle to Li Zicheng's army. For hours, both sides attempted to defeat the other through frontal assaults, both sides to no avail. As Dorgon saw the intense combats take place, he believed that both Wu Sangui and Li Zicheng had committed all their forces into battle against each other, so he sent his Manchu force, which was still kept out of action, around Wu's right flank so that they will charge toward Li's left flank. The Manchus had total surprise, for they were also aided by a blinding sandstorm. The men of Shun were totally surprised by the Manchu charge, and as Li Zicheng fled the battlefield, the Shun forces were routed. The end of that sandstorm saw the demise of not only an army but also an empire as well.

The accounts of Hsi and Wakeman should be looked at carefully because they proposed the two most contrasting versions of the battle. Hsi's account suggested that Wu Sangui was forced to surrender to the attacking Manchus because they attacked him while he was away doing battle with Li Zicheng, while Wakeman, and in some aspects the chronicles of The History of Ancient Wars in China too, wrote that Wu Sangui voluntarily submit to the Manchus.