It really isn't too far fetched to consider that something like this might occur. As far as a realistic scenario, I think a more plausible one might be something like this:
2005-2006 - G7 & IMF call on the U.S. to devalue the dollar significantly, in order to "ease global imbalances." Policymakers agree with this move, primarily to make the U.S. more competitive with China.
23 March 2006 - Federal Reserve stops reporting the M3 statistics, which is the broadest indicator of how much money is in the system.
2006 - Dollar depreciates by 35%.
17 February 2007 - G7 & IMF urge the U.S. to continue the dollar devaluation, by "as much as" 20% over the course of 2007.
(these things have happened. The rest is an exercise in probability/possibility)
2007-2010(?) - The U.S. continues to devalue the dollar. It loses half of its purchasing power, in real terms, compared to 2000 levels.
2007 - Concerned that the Republican party will get a sound beating come election time, looking to the 2006 elections as a foreshadowing, Bush decides that the only way to turn his party's fortunes around is with a quick win against Iran, again looking to the opening weeks of Iraq as a model. It is a gamble, and it fails. The U.S. is unprepared for what they walk into.
2008 - With an unpopular war on, liberal Democrats walk on the competition. Giulliani doesn't stand a chance against Clinton, who is related in the people's minds to "the good old days" under her husband.
2010-2015 - The economy plummets, as it has been warning for years that it was going to do. The people take to the streets like they did in the late 70's, like they did in the 30's, like they did in the 1890's. This primarily equates to a resentment against the massive immigration of the 90's and 2000's, and again just like in the 30's and 1890's, when there were race riots involving the Chinese in San Francisco, against Irish and Italian immigrants on the east coast, there are substantial anti-immigrant feelings amongst most of working-class America. Mexico and China, along with other nations such as Venezuela, Cuba, and Russia, start declaiming what is happening as human rights abuses and pressure the U.S. government to intervene.
2010-2015 - Clinton hails New Orleans of 2005 as a model of the successful use of the military in quelling civilian discontent. Troops are deployed (mostly through the use of the National Guard, what is left of it, at least at first) in every major city in the U.S. The action is welcomed in the east and northeast and on the west coast. The heartland west of the Mississippi and the entire southeast region simmer at what they see as signs of an oppressive government.
2012-2015 - A mistake is made. There is a civilian protest, and the military is there to break it up, like they have been doing for the last several years. This time rocks are thrown, and somebody in the crowd brought a gun. An officer goes down. The military opens up. There are some thirty people dead. Word gets around fast, there's been a massacre. What had been simmering leads to open revolt. The western states take the lead, Montana and Wyoming and Idaho, followed quickly by every single state in the western and southeastern regions. Independence has been declared. There is now open war.
2015-2016 - Seizing the initiative, Mexico declares that it will protect the rights of its citizens abroad. With the logistical aid of China it moves into New Mexico, Arizona, and California. These three states secede from the secession, declaring the new state of NorteMejico. Texas becomes a battleground state. Colorado becomes a battleground state. Utah decides to wait it out, and welcomes all, provided that they convert to Mormonism. Northern California secedes from Southern California. Iowa is immediately occupied by federal forces in a strategic effort to control the continent's food supply. Tanks rumble toward Nebraska and Missouri. The west coast is also occupied immediately, San Diego and Seattle and San Francisco primarily, with spokes of control radiating from those points, where the federal forces have a substantial naval and military presence.
2016-? - With a shooting war on at home, United States troops are recalled from everywhere on the globe. Iran appears victorious, and is de facto appointed the head of a new middle-eastern alliance, immediately threatening the "zionists" of Israel. China destroys the Japanese Navy with a stroke and invades Taiwan, then blockades Japanese ports. South Korea sues for peace before war can be declared, and China accepts with the caveat being a unified Korean puppet-state. In a bid to end the population pressures on the Sino-Russian border, neo-communist Russia under Putin agrees to divide resource-rich Alaska with the Chinese, and the nations jointly invade the state against little to no opposition. Clinton repeals the term-limit law placed in effect after Roosevelt.
(and for a lttle more fanciful imaginings, but still not too terribly far-fetched)
2016-? - The heartland, feeling the brunt of the war, appeals for outside aid. The only nation to openly side with them is China, again declaiming the human rights abuses of the federalist forces. Already being in a logistical position within Mexico and NorteMexico, China deploys its troops north, along the west coast, nominally in order to counter the federalist forces there. High-positioned generals are overheard laughing at the stupidity of Americans. The Chinese aren't leaving. Canada has been officially neutral throughout so far, but have provided material (food and medicine primarily) support to the heartland. This has caused hard feelings with federal forces, and there have been minor skirmishes, primarily in the Great Lakes region. Quebec sees this as an opportunity, and secedes, severing the Newfoundland area from the rest of Canada. Concerned with the growing Chinese and Russian presence in the north- and southwest, the majority of the forces that Canada can counter Quebec with come from the eastern provinces. Quebec appeals for aid, and both the EU and federal forces take up positions on the east coast of Canada, nullifying the threat. Canada is now by default allied with the heartland. An alliance of Central- and SouthAmerican nations, headed by Cuba, Venezuela and Panama, invade Southern Florida, claiming it is Hispanic by heritage and demographics.
and so it goes...
