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The Battle of Shanhaiguan paved the way for a quick Manchu occupation of North China, for Li Zicheng could never seize the initiative or halt the enemy's momentum again after the battle. In the decades of Nurhaci and Huang Taiji, the Manchus battled a determined, if constantly fumbling, Ming enemy for slitters of territories in northeast China, but in less than two years after Shanhaiguan the Manchus occupied territories that are many times larger and richer than their original kingdom. In comparison, the later Manchu conquest of South China required more time, faced more difficulties, and ultimately forced the Manchus to rely more and more on their Chinese allies.

Chinese historical sources had often depict Wu Sangui as a fool, for allowing the Manchus at first as equal allies but only to have the Manchus to push him to the remote southwest China. With hindsight on their side, they are correct. Although there are insufficient sources on Wu Sangui's mindset in 1644, one can only estimate what his strategy was. Wu Sangui probably hope to use the Manchus as mercenaries against Li, and once the Shun and Qing dissipate each other's strength, Wu Sangui would be able to establish his kingdom in North China on the carcasses of both the Manchus and Li Zicheng. Wu Sangui probably did not think that allowing the Manchus through Shanhaiguan was a traitorous act, for this had been done before: Li Yuan relied on the Turks' aid in his bid for empire, and at the dying days of the Tang Dynasty the emperors had often relied on the Uighurs to prop up their throne. After all, Wu Sangui probably guessed that the Manchus would eventually leave China, as the Turks and Uighurs did before. In this aspect, he was not alone: the courtiers in Nanjing, in the process of setting up a successor South Ming state after the death of the Chongzhen Emperor, also hoped that the Manchus would enter China only to have them bleed themselves white in battle with Li Zicheng. With hindsight in our knowledge, we can now see that in April and May of 1644 Wu Sangui's biggest mistake was misunderstanding the Manchus, in particular Dorgon, who's ambition was to occupy China.

Li Zicheng's speedy march to Beijing showed Li Zicheng and his men's strategic and tactical brilliance, yet at the same time they failed to polish the last piece of the march to Beijing that eventually proved their downfall: the failure to combat the combined force of Wu Sangui and the Manchus. Why Li Zicheng failed to prepare for war against the combined Sino-Manchu force is still a great mystery, for Li Zicheng should have been aware of the Manchus, especially if he did send the proposal to the Chongzhen Emperor. However, there could have been another aspect of Li Zicheng and Shanhaiguan: perhaps in late May when Li Zicheng led his army toward Shanhaiguan, he was attempting to destroy Wu Sangui's smaller force before it can join with the Manchus, after all Dorgon was still at Ningyuan, 100 miles away from Shanhaiguan, the day before the battle. That might perhaps explain Tang Tong's role in the battle, that Li Zicheng sent him on a flanking march out of the Great Wall to prevent Wu Sangui from escaping into Manchuria, and he was surprised by the appearance of the Manchus at Sha He because he thought the Manchu main force was still many miles away. If this was what shipwrecked Li Zicheng's battle plan, then that means that Dorgon's decision (not to mention Hong Chengchou's caution) to force-march his men to Shanhaiguan sealed the fate of the Shun state.

In the events of Shanhaiguan, Dorgon's success was more due to his cautiousness and carefulness rather than any audacious strategy. Accept Wu Sangui's surrender was not something new to the Manchus, so simply Dorgon had to make sure that he controls Wu Sangui and not the other way around. That is the reason why he ordered Wu Sangui to do battle with the enemy first, so that would diminish the Ming army's strength and thus making Wu Sangui dependent on the Manchus. Instead of tricking the Manchus and Shun forces into killing each other, Wu Sangui was forced to do battle with Li Zicheng to his disadvantage. Dorgon's conduct of the battle was probably the best conduct for the Manchus, because had he attacked Miyun instead, there was the chance that Li Zicheng could have quickly eliminated Wu Sangui at Shanhaiguan and then quickly march back to defend Beijing. Besides, Shanhaiguan was, usual a Sun Zi description, a "fatal terrain": an unknown terrain that is very far away from his main base, while it is a familiar terrain close to the enemy's interior lines.

The Battle of Shanhaiguan also produced another loser: the Southern Ming. The officials in Nanjing, who were trying to establish a successor government after the fall of the capital, hoped that the Manchus and Li Zicheng would destroy each other. The Manchus' swift and overwhelming victory over Li Zicheng dashed away the Southern Ming's plan to "use barbarians against bandits." Had Li Zicheng came out of the battle with better results, chances are he would be able to retain enough military strength to bloody the Manchu's march toward an empire, especially during his last stand in Xi'an. Once the Manchus swallow up Li Zicheng's infant Shun state, the Manchus would find the politically divided and confused Southern Ming as their next prey.

In the end, the most interesting aspect of the Battle of Shanhaiguan was that the situation in May of 1644 was similar to a game of Risk with three players. Dorgon, Wu Sangui, and Li Zicheng all had powerful armies (although one can argue that Wu's force was probably the smallest of all three), and when all three of them came to Shanhaiguan, any combination of two against one could probably vanquish the odd one out, as was said in Angela Hsi's "Wu San-kuei in 1644: A Reappraisal." Therefore, curious history readers might wonder, what if Wu Sangui did not join the Manchus?

Had Wu Sangui surrendered to Li Zicheng, his forces, with support from the new Shun state, could probably hold back further Manchu attacks on the Great Wall, as Wu Sangui had repulsed the Manchus' previous attacks on Ningyuan. Not only were Wu's Ming soldiers seasoned-veterans in combating the Manchus, but the Shun Army was also a strong force in cavalry, infantry, and artillery. Had Dorgon blundered into Shanhaiguan with Wu allied with Li, a major Manchu setback could have ended with the Shun instead of the Qing as the successor to the Ming.