War with China could come from many reasons, most likely something to do with Taiwan or a snowballing Korean conflict. I guess if Japan and China went at it as well, not to mention all the other disputed territories around the area I imagine it would be fairly unlikely though as both countries make so much money off of each other that it would have to be something pretty big for either side to want to really jepordise that, at the moment China and the US pretty much rely on each other to keep each of their economies going. If the US stopped buying Chinas cheap junk then China loses it's income and if that happens then all the US stocks, currency and bonds China has been buying and holding (keeping the US dollar afloat) plumments in value taking the US dollar with it.
It would come down to economics and who had access to the most resources, perhaps the most important being oil and metals. A lot of stuff is made in China, but whether they have enough material to sustain a long term war is debateable. Then again the US is also not as abundant in resources or manufacturing as it once was, though it could probably call on more allies if the need arose.
The other critical factor for China is it's population, it's huge, and male dominated, however it's also ageing very rapidly(just about fastest in the world) thanks to their one child policy, unless this war happens soon, they wont have quite the massive amount of disposable manpower that many people think. Also consider that the US is one of the few western countries to have retained a positive population growth and a long term war of attrition isn't so obviously in Chinas favour.
A more interesting Scenario would perhaps be India vs China, a more even match, in fact I would probably back India. Or a bizarre, but fairly evenly matched one that someone mentioned was Australia vs Canada (god knows where they would fight though, Hawaii? :laugh
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