Under the pretext that his concubine Chen Yuanyuan was being taken over, Wu Sangui united with the Manchu Qing to oppose Li Zicheng's peasant army. Enraged, Li Zicheng beheaded Wu Sangui's father, Wu Xiang, and his family, causing Wu Sangui to completely fall to the Manchu Qing. In the end, Manchu Qing and Wu Sangui united to defeat Li Zicheng. After the Manchu Qing Dynasty moved its capital to Beijing, Wu Sangui was made the King of Pingxi. Although the Manchu Qing initially did not trust Wu Sangui. Therefore, after joining forces with Wu Sangui to eliminate the remnants of Li Zicheng, he was immediately transferred back from the front line and sent to Guanwai. On the surface, he was out of the town of Jinzhou, but in fact he was placed under the eyes of The Manchu Qing and supervised.
The situation in Liaodong was temporarily brought under control, and the Qing army could not penetrate Shanhaiguan. The Chongzhen Emperor was relieved, but at this time, Li Zicheng's army was like a bamboo, and he never stopped at all. All the armies of the Chongzhen Emperor were defeated by Li Zicheng, and everywhere there were generals who rebelled against him. The Chongzhen Emperor knew that things were not good, and if Tongguan was breached, then the capital would be in danger. Therefore, the Chongzhen Emperor ordered Sun Chuanting to guard Tongguan, and Sun Chuanting claimed that Daming's last hope, if he could not keep it, presumably no one could hold it.
Between 1640 and 1642, a major war broke out between the Ming Dynasty and the Qing army, the Battle of Songjin. In this battle, the Ming Dynasty invested about 130,000 troops and the Qing army strength of about 110,000 troops, which ended in a major defeat of the Ming army. For Huang Taiji, this was a glorious period in his life, the most splendid battle of his military career, and a typical example of siege and reinforcement in Chinese military history. But for the Ming Dynasty, which was already in turmoil, it was like worse, the Ningjin Defense Line, which had been operating for many years, was destroyed, and the elite of the nine sides that had been built with national strength was lost, which was an unbearable defeat in life.
Wu Sangui is not a pure general, he is a cold-blooded politician. The attitude toward the Ming Dynasty depended on what the Ming Dynasty could give him, the more he gave, the more loyal he was, and when nothing could be given, loyalty was a joke. Given the circumstances, we cannot say that his choice was wrong. However, there are always some things that cannot be measured by right and wrong, but must be insisted on, and can the enemy be surrendered when the country is broken? Wu Sangui felt that he could, but many people felt that he could not! I'd rather die than fall.