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Qualitatively, Wu Sangui probably had the best field force in the entire Ming Empire. It was part of the Grand Army of Hong Chengchou, so it was at least very well equipped in all fields (cavalry, artillery, and infantry). The fact that it had survived after the Battle of Songshan and repulsed Manchu assaults on Ningyuan afterwards suggests that Wu Sangui's forces were roughly the equals of the fierce Manchu warriors.

Many historians wrote that Wu Sangui had about 40,000 troops, and later was able to raise 50,000 militiamen. The author has doubts about those numbers, for during April and May of 1644, Wu Sangui only control the tiny territory between Ningyuan and Shanhaiguan, which was a military frontier zone and so was not heavily populated. Therefore, it would seem difficult for Wu Sangui to raise so many troops at such a short time in that area, and even if he was able to raise that many militiamen, they were probably men of a variety of age, physical condition, and military expertise, which means that at best they would have been human fences in battle against the combat veterans of the Shun Army. Wu Sangui could have 40,000 regular troops, but it seems strange that the Ming would concentrate so many troops in just one garrison after sustaining the tremendous losses from the Battle of Songshan. The History of Ancient Wars in China, published by the Republic of China Armed Forces College, said that after the battle at Shanhaiguan Dorgon dispatched Wu Sangui and his 20,000 infantrymen and cavalrymen to chase after Li Zicheng. In addition, in the days between the fall of Beijing and the Battle of Shanhaiguan, Wu Sangui was reluctantly courting the Qing court to aid him against Li Zicheng (according to the letter that Wu Sangui wrote to Dorgon), which suggested that Wu Sangui had insufficient strength against Li Zicheng. It is therefore probably best to assume that Wu Sangui's Ningyuan garrison had between 20,000 to 40,000 troops, with 30,000 being the most likely number.

The Manchus, who inhabited in the present-day provinces of Jilin and Heilongjiang, were the descendants of the Ruzhens (Jurchens), who conquered northern China and found the Jin ('jin' means gold) Dynasty in the 12th century. Just like their Jurchen ancestors, the Manchus were expert horse-mounted archers.

The founder of the Qing Dynasty was a Manchu chieftain named Nurhaci (1559-1626). At the age of 25, Nurhaci took over the leadership of one of the four Manchu tribes. After he united the four tribes of the Manchus, Nurhaci legitimized his regime by founding the Jin Dynasty (or Later Jin, since the first Jin Dynasty was the one founded by the Jurchens in the 12th century) in 1616. He then embarked on his foreign conquests. He invaded his neighbor tribe the Yehe, which led to a war with the Ming Empire in 1618. Compared to his tough horsemen, the Ming soldiers were ill trained and low in morale, which led to the decisive defeat of the Ming army at the Battle of Sarhu in 1619. Very soon, most of the cities of Liaodong (which was already predominantly Chinese by that time) were incorporated into his new domain. Confident after his successes, Nurhaci pressed on toward Beijing, but when he attacked the Chinese garrison at Ningyuan in 1625, the garrison commander Yuan Chonghuan skillfully used his cannons against his army and fatally injured the Manchu khan himself. So Nurhaci died disgruntled and without reaching his goals, but his ambitions would be carried on by his descendants.

Huang Taiji passed away in 1643, and for a moment a succession crisis threatened the stability of the nation, since Huang Taiji did not designate a successor at the time of his death (just like his father, Nurhaci). One faction supported Huang Taiji's 37 years old son Haoge, while another faction had Dorgon as the candidate for the throne. The Dorgon faction had the support of three banners (Dorgon's full brothers Ajige and Dodo each had a banner, and Dorgon led another one himself), but not powerful enough to eliminate Haoge. In the end, the tribal leaders, including the late Huang Taiji's Mongol wife the Xiaozhuang Empress, proposed a compromise by having Huang Taiji's 6 years old son Fulin as the emperor, with Dorgon and Jirgalang (another son of Nurhaci) as co-regents until Fulin reach maturity. In reality, Dorgon took the reins of power, and after he settled the succession crisis, he began to move against the dying Ming Empire.