山海关
Soon after the emperor had called for help, Ming general Wu Sangui had left his stronghold of Ningyuan north of the Great Wall and started marching toward the capital. On 26 April, his armies had moved through the fortifications of the Shanhai Pass (the eastern end of the Great Wall) and were marching toward Beijing when he heard that the city had fallen, whereupon he returned to the Shanhai Pass. Li Zicheng sent two armies to attack the Pass but Wu's battle-hardened troops defeated them easily on 5 May and 10 May. Then on 18 May, Li Zicheng personally led 60,000 of his troops out of Beijing to attack Wu. At the same time, Wu Sangui wrote to Dorgon to request the Qing's help in ousting the bandits and restoring the Ming dynasty.