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While peasant bandits like Li Zicheng and Zhang Xianzhong established their prestige (or notoriety) and empires, up until 1644 Wu Sangui remained an unknown frontiers soldier. A native of Liaodong (of the present-day province of Liaoning), Wu Sangui was the son of the frontiers general Wu Xiang. Descriptions of Wu Sangui or his military career were scanty, although it is known that Wu Sangui was the Ningyuan garrison commander and one of Hong Chengchou's eight subordinate generals when the Ming court sent Hong to lift the Manchu siege of Jinzhou in 1643.

For several years, General Zu Dashou had held the forward fortress of Jinzhou in the face of Manchu ambitions, but in 1643 Huang Taiji, the eighth son of Nurhaci and the second leader of the Manchus, renew his assault on Jinzhou. Not only did Huang Taiji retain his father's martial vigor, but he also held tremendous political wisdom and ruthlessness. He consolidated his absolute power over the eight Manchu banner belies (princes) (4) by taking control of three out of the eight banners. He faced potential rivalry from Nurhaci's older sons and nephew Daisan, Manggultai, and Amin, and he subdued all of them. Later on, in 1636 he gave his people a new identity by proclaiming the establishment of the Qing state and renamed his people the Manchus. At the same time, he invaded Korea and made it a tributary state of the Qing, introduced captured Chinese and European cannons in his army, took the Ming fortress of Dalinghe from Zu Dashou, defeated the Chahar Mongol khan Lindan (or Lighdan) and absorbed the Chahar Mongols, repeatedly raided northern China, and created Han and Mongol banners.

The Manchu raids into China brought them treasures, food, and live stocks, but the logistical difficulties along the circuitous invasion route through the Inner Mongolian deserts (in order to bypass the Ming garrisons of Shanhaiguan, Ningyuan, and Jinzhou along the Bohai littoral) made it difficult for the Manchus to hold onto their territorial conquests. The Shanhaiguan corridor remained the best invasion route for the Manchus and therefore Huang Taiji needed to eliminate those Ming fortresses, the first one being Jinzhou. To save the vital fortress, the Ming court sent an army of over 130,000 men under Hong Chengchou to lift the siege. Unfortunately, in a series of skirmishes, Huang Taiji's Manchus massacred the flower of the Ming Army. First, Manchu cavalry raided the Ming's granary in the rear, and when the Ming Army retreated after it ran out of food, Huang Taiji placed ambush forces along the Ming Army's retreat routes and massacred the retreating Ming soldiers at night. In the Battle of Songshan, the Ming lost over 50,000 troops and a large amount of weapons. Very soon, Jinzhou capitulated and Hong Chengchou became Huang Taiji's special prisoner.

Wu Sangui and Tang Tong were among the few Ming commanders in the rear who were able to escape the Manchu ambush. Afterwards, the two return to their original stations in Ningyuan and Miyun, respectively. When Li Zicheng edged toward the capital, the emperor ordered them to save the capital. Tang Tong arrived in time, only to be overwhelmed by the Shun forces, but for some reasons Wu Sangui did not get to the capital in time.