They effectively own a significant part of the World financially and could easily ruin the US and big Parts of Europe by simply claiming back all their Money they loaned to Western State Banks and selling their Shares in Companies at once. The whole economic Systems would immediately and totally implode.
Though technically true, I don't see why on Earth they would ever want to do this...they have been keeping the Renminbi artificially low in order to practice dumping with all their cheap tat in Western economies, financed by all the money they lend out; they would totally shoot themselves in the foot if they ever decided to do this!
And if the US and China ever do square up to each-other, the US (and the West) could just give two fingers to their debt obligations.
I have to say, I'm with Lonewolf; Chinese aggression seems to be advancing rapidly...we can think of it as nonsense sabre rattling, but I see trouble brewing.