A Chinese civil war would spill all over the place. Look at the chaos surrounding the Syrian civil war and scale it up by at least an order of magnitude. A billion refugees trying to flee across the borders mostly in the east (where population densities are highest) would turn the surrounding countries into giant refugee camps, unless they somehow manage to close their borders.

North Korea is so fragile that it would follow shortly thereafter. South Korea (a US ally) would have a huge mess on their hands.

Japan (another US ally) has got an ocean to protect it, but given the huge shipping volume in the region, shiploads of refugees would try crossing anyway.

So that means that the US would have even more reason to intervene than in other conflicts.

I don't know how much Russia cares about it's eastern territory, but it's unlikely they'd sit around doing nothing.

By now there are 4 countries with nuclear weapons involved, and I haven't even mentioned India and Pakistan, who usually don't appear to be the interventionist type, but they have a common border with China. Who knows how they might react.

Basically, any war involving China is bound to be WW3 levels of bad.