Wu Sangui
Li Zicheng then led rebel forces numbering some 200,000 to confront Wu Sangui, at Shanhai Pass, a key pass of the Great Wall, which defended the capital. Wu Sangui, caught between a Chinese rebel army twice his size and a foreign enemy he had fought for years, cast his lot with the familiar Manchus. Wu Sangui may have been influenced by Li Zicheng's mistreatment of wealthy and cultured officials, including Li's own family; it was said that Li took Wu's concubine Chen Yuanyuan for himself. Wu and Dorgon allied in the name of avenging the death of the Chongzhen Emperor. Together, the two former enemies met and defeated Li Zicheng's rebel forces in battle on May 27, 1644.