Beat me to it.
My thoughts exactly.
China pre-emptively attacks North Korea before NK can do anything and before the US can do anything. China works with S. Korea to unify and support transition N.K. to an indenpendent Korean state. The World claps their hands at China. China and Korea become allies. Unified Korea looks at US and commands us to get our troops out of S. Korea since they decide we are worthless.
Taiwan observes the US weakness in this situation and decides that the US is not the long term solution either. They decide to join Mainland China...
............