I don't think anyone is doubting that China is going to overtake the U.S. in terms of GDP. I also think that is obvious that the U.S. will lose some dominance in terms of power projection but will still remain regionally dominate over the entire Americas and will not be threatened with domestic invasion or attack. In our lifetimes Afghanistan and Iraq will be looked on as defeats as governments unfriendly to the U.S. will emerge in both countries.
The U.S. has failed to complete its military objectives in Vietnam, Afghanistan and Iraq. I think this is a good thing because we shouldn't be nation building anyways and I'd prefer to see the U.S. act in a more restrained matter militarily akin to the Gulf War. The U.S. is going to have to learn to adapt to a low GDP growth scenario of 1-3% and Europe is going to have to adapt to a 0-2% scenario.