I think it is possible, but it would essentially have to involve every major power on earth dogpiling on the United States in an all-or nothing attempt re-divide the world's spheres of influence. But I think the US is just too savvy and has too many hands up too many skirts in the world for that to happen. The US has military control over Afghanistan and political control over Iraq, making both nations viable staging points for an invasion of Iran. Iran and Israel are at the brink of war and we bomb Pakistan every day. South America is pushing out US economic and military influence, which has provoked the US to build up military forces in USSOUTHCOM. The North African and Middle Eastern Countries of Tunisia, Egypt, Libya, and Syria have thrown off or are throwing off US influence with fresh new regimes. The US has created USAFRICOM as a military theatre in anticipation for conflict there. I think that if the Arab and Latino uprisings against US hegemony prove to be successful (and the US does not face a civil war of its own amid economic turmoil), we could expect to see similar trends spread into Europe and East Asia--our two prizes from the second world war. Those are two regions that the US will never give up without a fight. Japan and China hold all of our debt. Southeast Asia holds most of our manufactures. We economically compete with Europe and more of their companies are establishing themselves here. Strange that World War III would again center on Europe and East Asia.
There is no doubt that the US is the greatest threat to peace in the world right now, and I hope it can be diffused by peaceful means. In the future, I think regional spheres of influence will be held by confederated nations, like the EU. East Asia, the Arab Nations, and South America will probably all have something similar. I could see most Anglo-Americans eventually emigrating away to Europe or elsewhere and leave the US as a Latin-American country by the end of the century. Anglo-Americans will ultimately go where the money goes.