 The largest remaining Ming fighting force in North China at the time of Beijing's fall was Wu Sangui's 40,000-man frontier force, who had abandoned the Ningyuan garrison to come to the emperor's aid.Halfway to Beijing, Wu received news of Chongzhen's death, so he went back to garrison the Shanhai Pass, the eastern terminus of the main Great Wall line.He and his men were now caught between the rebels within the Great Wall and the Manchus without.