America's only "need" to be in Afghanistan was a short-sighted political one, as usual demanding a standard of others they never force on themselves. As any person realised who has even the sketchiest knowledge of the are or wars in general, it's easier to get into a war than out of one.

China is diversifying how it purchases goods and I imagine the long term aim is to reduce its need of the dollar. Nothing too soon, as else the value of their existing bonds will decrease.

America needs its currency to be the world's reserve currency as its current account deficit requires people to continue buying today's debts to pay for yesterday's. Defaulting would probably mean people look for other leading currencies. Personally I'd go for the Rinimbi as the Euro has weak underpinnings and Japan is celebrating almost 20 years of near recession.