When Wu Sangui opened the fateful gate of Shanhai Guan to the Manchu, he had his reasons. As an imperial general, the rebellion from Li Zicheng seemed the most urgent. And allowing the Manchu in to 'help' didn't seem so bad.Such a public message means that this is now a life or death battle. Just a reminder that for a person facing potential death, there might be no moral bottom line, no family, no country, no values, no anything. Said person will do everything he can to win the battle in order to not die.
Russia is now at a potential historic turning point
Yet opening the gate sealed China's fate. The new dynasty led to the stagnation of the empire and the century of humiliation.