Li Zicheng then led a coalition of rebel forces numbering 200,000[a] to confront Wu Sangui, the general commanding the Ming garrison at Shanhai Pass. Shanhai Pass is a pivotal pass of the Great Wall, located fifty miles northeast of Beijing, and for years its defenses kept the Manchus from directly raiding the Ming capital. Wu Sangui, caught between a rebel army twice his size and a foreign enemy he had fought for years, decided to cast his lot with the Manchus, with whom he was familiar. Wu Sangui may have been influenced by Li Zicheng's mistreatment of his family and other wealthy and cultured officials; it was said that Li also 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.

How big was Li Zicheng's army?