www.China-Defense.com

Historians have two views as to why Wu Sangui did not arrive to the capital city to save it from the Shun forces. One view holds that Wu either outright defied orders or deliberately abandon the capital by advancing slowly. The other view is that the Chongzhen court belatedly notified Wu so that he did not have enough time to reach the capital. The author believes that the second theory is more plausible, because evidences suggest that the Ming government's intelligence service was deficient in following Li Zicheng's progresses and so it seemed that at the time the Chongzhen court was surprised by the speed of the Shun advance, not to mention that the Chongzhen Emperor was reluctant to re-deploy the frontiers forces, which would leave the Great Wall vulnerable to the Manchus. In addition, there are few motivations for Wu Sangui to defy order and remain in Ningyuan, for his family was in the capital and having the capital city under Shun control would leave him sandwiched between the Manchus and the Shun. Finally, Miyun is much closer to the western approaches of the capital than Ningyuan, so in any case it was most likely that Tang Tong arrived to combat the Shun Army before Wu Sangui was able to. Whatever the situation was, by the time Li Zicheng strolled into the imperial palaces of Jingcheng, Wu Sangui stopped in his march toward Beijing and bivouacked in Shanhaiguan.

Historians also have a variety of views toward Wu Sangui's motivation in joining the Manchus and opposing Li Zicheng. The popular Chinese notion was that Wu Sangui was about to surrender to Li Zicheng, who held his father captive, when he discovered that Li Zicheng took his beloved concubine Chen Yuanyuan. Therefore, the angry Wu Sangui vowed that he would vanquish Li Zicheng and thereafter asked the Manchus for aid. However, records and notes about the famed beauty Chen Yuanyuan were scanty, most of them came from either court writers or common legends. In addition, it was probably easier for Ming loyalists to answer to the stigma of Manchu rule by blaming it on one woman, instead of bearing the truth that Manchu conquest was aided by the citizens and officials of the Ming. In any case, whether the legend of Chen Yuanyuan is fact or fiction, it seems that in the spring of 1644 Wu Sangui's actions were motivated mostly by his personal ambitions. At that moment, Wu Sangui had two choices, surrender to either the Manchus or Li Zicheng, or do nothing and possibly allowing the former two forces to join together and eliminate him. Had Wu Sangui surrendered to Li Zicheng, he would probably gain riches from Li but would otherwise remain a subordinate general to Li Zicheng, no more powerful than Tang Tong or Bai Guangen (Bai Guangen served under the late Sun Chuanting and joined Li Zicheng when Li overran Shaanxi). On the other hand, serving the Manchus would probably allow him to garner personal power, for there were already many former Ming officers now serving the Manchus who hold personal, hereditary power (for example Li Yongfang, Kong Youde, Shang Kexi, and Geng Jimao). Perhaps just as important was reputation, for Wu Sangui might be afraid to be demonized by Chinese of all classes if he surrenders to the man who was responsible for the Ming emperor's death. If Wu Sangui was ambitious (and risking his feudatory powers by attempting to topple Manchu rule suggests ambition), he would want to advance his political and military status, but he cannot do so if he was demonized by the Chinese populace. At that time, it seemed that the subjects of Ming hated Li Zicheng more than the Manchus, so surrendering to the Manchus might be better than surrendering to Li Zicheng.

Whatever Wu's reasons were, he opposed Li Zicheng and decided to lead his army against Li. However, at the same time a Manchu force under the command of Prince Regent Dorgon advanced toward Ningyuan. Caught between two large forces, Wu proposed to surrender to the Manchus in order to eliminate Li Zicheng.