When the Chinese crossed the border during the Korean War, the US government threatened to use nuclear weapons if they didn't retreat in three days. Mao, thinking the Americans were bluffing and that the Soviets would back him, refused. In four days, Dalian, Harbin, Bei'an, and Changchung in Manchuria were hit by American nuclear strikes, and the Soviets promptly washed their hands clean of Mao following American threats of further nuclear strikes, leading to a panicked retreat by the Chinese from Korea and the beginning of the PRC's slow slide to self-destruction over the following decades.
But what if Mao was right, and the Americans were bluffing? What if the Americans didn't bomb Manchuria?