...is exactly the involvement of China.
PRC is a nuclear power; they have about 300 nukes and a "defense-only" use policy. They will not like a potential nuclear conflict close to their borders.
On the other hand, even in case of an overt conflict in the Korean peninsula, I doubt that the US will be the first to use nukes. North Koreans are fool enough to try to use their low-yield nukes (they are estimated to have about 12), but they do not have the delivery capabilities to nuke US soil. They might try to use them on invading ground forces, or against civilian targets in South Korea and Japan, but in that case, the US would retaliate with nukes, and China would remain out of the game.
The reason is simple: China has way too much to loose economically from direct involvement in a conflict between South Korea/USA and North Korea. PRC makes more $$$ yearly by trading with Japan, Taiwan, South Korea, the US, and the so-called "western world" in general than it would make in a decade of sales of armaments and basic commodities in exchange with precious and rare minerals (pretty abundant in DPRK soil) to North Korea. They will not let Kim Jong-Whoever to spoil everything. There is simply too much money to loose. And China might be communist as it concerns the form of government, but they are probably the most capitalist country in the world as far as it concerns the market economy...
And, guess what happened today? DPRK has (in its own very peculiar form, obvuiously) "apologized" for the loss of "civilian lives". Is it a case that the DPRK ambassador in Beijing has been "summoned for talks" by the Chinese government yesterday? No. They are being forced by China to de-escalate. There will be no conflict, not now. Too many economic interests are at stake in the region. The 2010s are not the 1950s.
"It is criminal to teach a man not to defend himself, when he is the constant victim of brutal attacks. It is legal and lawful to own a shotgun or a rifle. We believe in obeying the law." -- Malcolm X
"We (atheists) act in good conscience because we believe in moral principles, not because we expect a reward in Heaven." -- Margherita Hack