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By the early 1640s, Li Zicheng has established a strong base in Henan. After the destruction of Kaifeng during one of his sieges, the last threat to Li Zicheng's Henan base was the Ming general Sun Chuanting, who organized his own well-trained militia in the western provinces. However, the Ming government ordered Sun to attack Li Zicheng against the former's advice, and in a serious of battles in 1643 Li Zicheng eliminated Sun as the last defense of the Ming Dynasty. Once Sun Chuanting was eliminated, Li Zicheng began his quest for imperial power.

Originally a trader, Zhang Xianzhong led peasant rebel groups against the government at an early age. After a series of defeats at the hands of local government forces, eventually Zhang Xianzhong succeeded in establishing his personal fief "Da Xi Guo" (meaning the great western state in Chinese) in the western Chinese province of Sichuan. In 1644, he settled down as the despot of Sichuan, busying himself with arresting and murdering pretended or real enemies in his kingdom. Thus, he alienated his subjects as quickly as he had won their hearts over. In a little over a year, the abused people of Sichuan would welcome the Manchu invaders and end Zhang's career and reign of terror.

With his army routed in the previous Henan battles, Sun and his generals were in no position to halt Li Zicheng when his forces enter the Tongguan (3). Li Zicheng quickly enter Xi An, which he established as the capital of his new state, Da Shun Guo (the Great Shun State), or the Shun Dynasty. His generals and he quickly pacified Shaanxi, after which, they began the planning of the next offensive: the capture of Beijing.

From northern Shaanxi, the Shun army marches toward Taiyuan, the provincial capital of Shanxi province, which quickly surrendered to his forces. The loss of southern Shanxi was a huge blow to the Ming government, for it had depended on the natural defenses of the lower Huang He (Yellow River) to protect it from the bandits of the western provinces. Thereafter, Li led his main force toward northern Shanxi while a smaller Shun army threatened the capital from southern Shanxi via the Gu Pass. According to Li's master plan, the smaller Shun army would threaten the capital from the south through Baoding, while Li's main force tread through the Great Wall garrisons in northern Shanxi and Beizhili (the northern capital district, now Hebei province) and attack the capital from the north.

Soon the Shun main force met heavy resistance from Ming government forces in the vital Ningwu Pass in northern Shanxi. The strategic pass connects the mountainous province of Shanxi to the Hebei plains that surround present-day Beijing, and therefore the capture of the pass was crucial to Li. The Ming garrison inflicted such heavy losses on the Shun forces that Li contemplated halting his advance for a while, but Li Zicheng quickly rejoiced when he found out that the other Shanxi garrisons agree to surrender to him. Without taking a short respite to lick its wounds, the Shun army passes through Datong and aim toward the Juyong Pass.