The American empire is probably going to last more than 10-20 years, unless the US does so badly in Iraq that it can only win with a draft. If a draft is imposed the most likely result is that it will cause a national revolt which will force the US to reduce its troops in Iraq, end the draft, and rely on EU & other nations troops to suppress the insurgency. That would be the beginning of the end of the American empire, since it would have to reach a power sharing agreement with other nations, leading to a sort of mult-national imperialism. Similar to how Greece '47 signaled the end of the British empire. It's possible that the crisis caused by the draft could cause the union to break up, but that would probably require a very large revolt that could not be pacified by the national government even by ending the draft. If that happens the states won't separate economically, they'll only separate politically. Some sort of "free trade area" and transnational authority could be used to achieve that. That will probably be the rule of thumb across the globe after the end of the empire; the global economy is too integrated for it to be in the interests of multinational corporations to split the world up into multiple competing empires. Barring revolution, it'll be the reign of free trade agreements and WTO or EU like authorities.