The Battle of Shanhai Pass, fought on May 27, 1644 at Shanhai Pass at the eastern end of the Great Wall, was a decisive battle leading to the beginning of the Qing dynasty rule in China proper. There, the Qing prince-regent Dorgon allied with former Ming general Wu Sangui to defeat rebel leader Li Zicheng of the Shun dynasty, allowing Dorgon and the Qing army to rapidly conquer Beijing.
Prelude
Rise of the Manchus
As the Ming dynasty declined, and the threat from northern enemies grew, Ming emperors saw the strategic value of Shanhai Pass and frequently garrisoned troops there, armies which sometimes reached up to 40,000 men. Under the rule of Hong Taiji (r. 1626–1643), the Qing were becoming more aggressive against the Ming. After an intermittent siege that lasted over ten years, Qing armies led by Jirgalang captured Songshan and Jinzhou in early 1642. The garrison of Ming general Wu Sangui in Ningyuan became the only major army standing between the Qing forces and the Ming capital in Beijing. In the summer of 1642, a Qing army managed to cross the Great Wall and ravaged northern China for seven months before withdrawing in May 1643, with prisoners and loot, without having fought any large Ming army.
In September 1643, Hong Taiji suddenly died without having named an heir. To avert a conflict between two strong contenders for succession – namely Hong Taiji's eldest son Hooge and Hong Taiji's agnate brother Dorgon, a proven military leader – a committee of Manchu princes chose to pass the throne to Hong Taiji's five-year-old son Fulin and appointed Dorgon and Jirgalang as co-regents. Because Jirgalang had no political ambition, Dorgon became the prime ruler of the Qing government.
Fall of Beijing
Just as Dorgon and his advisors were pondering how to attack the Ming, peasant rebellions were ravaging northern China and threatening the Ming capital of Beijing. In February 1644, rebel leader Li Zicheng founded the Shun dynasty in Xi'an and proclaimed himself king. In March, his armies captured the important city of Taiyuan in Shanxi.
Seeing the progress of the rebels, on April 5, the Ming Chongzhen Emperor (Zhu Youjian) requested the urgent help of any military commandant in the empire. Eager to secure the loyalty of his military elite, on April 11 he granted the title of "earl" to four generals, including Wu Sangui and Tang Tong (唐通). Tang Tong, the only one of these new earls who was then in Beijing, reorganized the capital's defenses and, with a eunuch named Du Xun (杜勳), went to fortify Juyong Pass, the last stronghold protecting the northern approach to Beijing. On April 22, the Ming court learned that Tang Tong had surrendered to Li Zicheng the day before and that the rebels' army was now in Changping, 65 kilometers northwest of Beijing.
Li and his army reached the suburbs of the capital on April 23, but instead of mounting a full-scale attack on the city walls Li sent the recently surrendered eunuch Du Xun to see Emperor Zhu Youjian, hoping to secure his surrender. Zhu refused. On April 24 Li Zicheng breached the walls of Beijing, and Zhu hanged himself the next day on a hill behind the Forbidden City. Zhu was the last Ming emperor to reign in Beijing.
Wu Sangui
Soon after the Emperor called for help, powerful Ming general Wu Sangui left his stronghold of Ningyuan north of the Great Wall and started marching toward the capital. On April 26, his armies had moved through the fortifications of Shanhai Pass (the eastern end of the Great Wall) and were marching toward Beijing when he heard that the city had fallen. He returned to Shanhai Pass. Li Zicheng sent two armies to attack the pass but Wu's battle-hardened troops defeated them easily on May 5 and 10. In order to secure his position, Li was determined to destroy Wu's army. On May 18 he personally led 60,000 troops out of Beijing to attack Wu. Meanwhile, Wu Sangui was writing to Dorgon to request the Qing's help in ousting the bandits and restoring the Ming dynasty.
Wu Sangui's departure from the stronghold of Ningyuan had left all territory outside the Great Wall under Qing control. Dorgon's Han Chinese advisors Hong Chengchou and Fan Wencheng (范文程) urged the Manchu prince to seize the opportunity of the fall of Beijing to claim the Mandate of Heaven for the Qing dynasty. Therefore, when Dorgon received Wu's letter, he was already leading an expedition to attack northern China and had no intention to restore the Ming. Dorgon asked Wu to work for the Qing instead. Wu had little choice but to accept.