let me get this straight
south korea is being defended by the US and there are US troops by it's border
north korea is ready to attack south korea
china is north korea's major ally
world war 3
We are China's chief economic partner next to the UK (who are our close allies). China will not start a war with the US because it'd be devastating to their economy, and we have better weapons and better soldiers. They may have numbers, but we have the M1 Abrams.
Chances are better that if NK tried to start a nuclear war, especially with the US, China would turn on them and then they'd be forgeted. The lunatic running NK will get loving steamrolled.
Even better is that the nukes Korea has are probably Hiroshima-size. We have H-bombs and Tsar Bombs a thousand times more powerful.