What if, simultaneously, NK invades SK and China invades Taiwan. What would the US do, led by a Biden administration soft on — and I would say naive about — China?
A NK invasion of SK would require the use of tactical nuclear weapons by the US to deter it (no way the US would send a large army into SK nor would there be enough time to mobilize and do that). But I don’t see the Biden administration willing to use tactical nuclear weapons. Same situation (need for the use of tactical nuclear weapons) with an invasion of Taiwan, which might actually be triggered by a successful first invasion by NK of SK because it would then embolden China — China would conclude that the US was actually a paper tiger and so not to be feared in their takeover of Taiwan. So the first invasion would lead to the second.
The lesson taught by history at Munich that led to the outbreak of a world war is simply that if one side is perceived to not be willing to protect it sphere of influence, that condition invites aggression by the other side.
Consider this: two such successful invasions would mean that the American sphere of influence in Asia would collapse. Japan, Philippines, Australia, New Zealand, and all of southeastern Asia would all become dominated by China because none of these nations would any longer trust the American defense. China would become the uncontested power throughout all of Asia. Therefore, what is obvious is that China would have a tremendous amount to be gained from a successful takeover of Taiwan — a much greater gain than just that takeover, i.e., a sphere of influence extending throughout all of Asia. Therefore, one should not dismiss a Taiwan invasion as unlikely — it is much more likely than not, imo.
These two situations — the status of both SK and Taiwan — have been simmering, tittering on the edge, and unresolved now for decades.