At the time of Beijing's fall to Li Zicheng, on 25 April 1644, Wu and his 40,000-man army—the most significant Ming fighting force in northern China—were on the way to Beijing to come to the Chongzhen Emperor's aid but then received word of the emperor's suicide. So they garrisoned the Shanhai Pass, the eastern terminus of the main Great Wall instead. Wu and his men were then caught between the rebels within the Great Wall and the Manchus without.
