Li Zicheng then led a collection of rebel forces numbering some 200,000 to confront Wu Sangui, the general commanding the Ming garrison at Shanhai Pass, a key pass of the Great Wall, located northeast of Beijing, which defended the capital. Wu Sangui, caught between a rebel army twice his size and an enemy he had fought for years, cast his lot with the foreign but 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.