Another way of looking at things - conflict in the military or economic sphere places a good deal of strain on the societies involved in that conflict. People are subjected to hardships because of the actions of their governments.
Stable, well-integrated societies can bear a great deal of strain if the majority of the population believe that the conflict is justified. As we have already seen US, Chinese, Indian and other societies are capable of generating formidable outbursts of patriotism and xenophobia given sufficient stimulus from external events and government propaganda. The question is, how long can they sustain their ‘enthusiasm’ when things get sticky? Despite much discussion of the ‘wussification’ of America, I find it hard to believe that American society will fracture more easily than that of the rivals in this hypothetical wrestling match.
China is, and will remain for the next 40 years or so, a third-world country struggling to eradicate poverty while suppressing the centrifugal tendencies generated by the enormous differences between its regions. The dictatorship which makes it so threatening is also its weakness inasmuch as it has no real legitimacy in its rule - unless it can maintain improvements in living standards it will have to face a billion citizens demanding accountability, and starting WW3 is not the way to generate prosperity.
India is, without wishing to be disrespectful to its citizens, nothing more than a paper boogy-man conjured by the paranoid. Despite all the talk of high-tech, offshoring and so on, the single biggest determinant of its economy is rainfall-dependent agriculture. One bad monsoon is enough to push India into recession and threaten millions with famine. Two bad monsoons in a row would topple it into depression and risk total breakdown. I can’t see India becoming a serious strategic power within the next couple of generations.
Europe and, as far as I can tell, Japan, have no appetite for world-straddling geopolitical might (tried it, didn’t like it, prefer to stay at home). The rest of the planet is and will remain split into small squabbling polities that won’t be able to mount an effective challenge to any major power.
So over the next half-century or so, I would expect to see the US maintain its leadership position, but with China and the EU rising to the status of secondary powers based mainly on economic clout. This would result in the US being Primus Inter Pares rather than leading by an order of magnitude, but still well clear of the pack.
My having written his is, of course, the cue for Muad’dib to arise and unite Africa, Poland and Patagonia in a global uprising agains capitalism, or for someone to discover a way of making self-aware military nanobots out of Pad Thai, and thereby prove that pontificating on the future is on a par with tap-dancing in a minefield…