The idea of a third world war on the scale of WW1 or WW2 is ridiculous. Honestly. World War 1 only went for so long because of the simple weaponry that required slow killing. World War 2 people literally dug trenches to avoid machine gun fire that would tear apart waves of people. You can't dig a trench to avoid a nuclear weapon. And don't pretend that the UN will keep that from happening either. When a country is on the brink of being dissolved by another, they will do anything to save themselves. With countries expending nukes at each other that's a nuclear holocaust not a war. There's no where to hide from the nukes we have today.
And on top of this, China has no beef with us. Their economy is fairly reliant on us. We purchase a large portion of their goods and such. They don't want to wipe us out, that's like shooting your best customer. You're not going to do as well without them, regardless of their tab.