Wu Sangui
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. After some deliberation, Wu Sangui decided to resist the new Shun regime, having heard that Li Zicheng had ordered Wu's family executed. On May3 and May10 Wu Sangui twice defeated the Shun vanguard led by the turncoat Tang Tong, but he knew that his force alone was insufficient to fight Li Zicheng's main army. Wu Sangui wrote to the Manchus for help, promising "great profits" if they assisted him in defeating the rebels. The Manchu prince-regent Dorgon (Hung Taiji had died in 1643) determined that this was the opportunity to claim the Mandate of Heaven for the Qing. Dorgon made clear in his reply that the Manchus would help Wu Sangui, but Wu would have to submit to the Qing; Wu had little choice but to accept.