[Aries Walker]

It would all come down to how and when the electricity shut off, because that is the key to mass communication. The government would try to restore (or retain) some sort of order, at least at first, probably by having police go door-to-door and sending out mass texts and phone calls through the phone companies. They would be working to restore services (though they would ultimately fail), to keep people calm, and to take care of the old, sick, and dependent.

The problems would start with the lack of gas - people wouldn't be able to get to work (especially with mass transit out), so many business would instantly close, and the bunker mentality would begin. The first life-threatening shortage would be that of water; the government, the military, and community programs would probably deliver some to each house while they could, but eventually they would run out as well. Grocery stores would have their bottled water seized not by mobs, but by the National Guard, who would stockpile it at their bases (possibly even raiding houses for more). Health and hygiene would be a big problem, disease would spread, and people - especially the old and very young - would die, a lot.

Panic, mass exodus, and violence would begin at varying rates, as some people realize what would be happening more quickly than others, so the roads would probably not clog any more than usual. Many people would remain in cities, waiting for the government to restore services and order, as their supplies dwindled. City and town police (whose numbers would be dwindling as many police would leave to tend to their own families) would have to shift their priorities from helping the sick and injured, to countering the growing mobs of desperate and starving citizens. Large sections of cities would burn to the ground as fire fighters, lacking water, would have no effective ways of combating them. Hospitals would close, as would prisons, whose inmates would likely be left to starve and die inside the walls, leading to some of the most horrific scenes in the nation. If this all happened within a heat wave or a snowstorm, the effects would increase exponentially.

However, the cities would not descend into barbarism immediately; neighborhoods would probably band together to form collectives. People with pre-existing relationships would mass together in one large house and form small intra-urban societies, protecting and providing services for each other. Swimming pools would become rainwater collectors, gardens would grow wherever they could, and collectives would probably even start to trade with one another. Some would do better than others, and as collectives failed, they would most likely merge with each other. The criminal element, however, would rely on what it knew, and before long intra-city protection rackets would grow, causing massive conflicts between them and police enclaves, many of whom would become little more than warlords. By today's standards, daily violence would be off the charts, but plenty would survive, though with a drastically more tenuous standard of living than before.

Meanwhile, in the rural areas, there would be a steady stream of city dwellers - light, at first, but then increasing in volume - heading out to live with relatives or friends, or to seize their own homesteads from the rightful owners who put up the least resistance (houses like Old Dog's would be avoided by bandits at all costs, especially after a band loses perhaps 25% of its population to the first volley of lead). Small communities would grow there as well, with larger plots of land to protect; horses - and people who could ride them - would become a major commodity in order to patrol the limits of property against squatters, interlopers, and outright thieves. The west would become in many ways the Wild West again, with fortified towns protected by 21st-century Sheriffs to provide a place to trade and support each other, but under constant assault by roving gangs; large highways would not be avoided because of road hazards, but because of these marauders for whom a single vehicle or small convoy is easy pickings for whatever supplies they carried. Also, many of these gangs won't necessarily be independent, but controlled by rural warlords, who may or may not have gotten their position from the remains of the Guard armory, corporate headquarters, or other pre-collapse institutions.

By six months, I should think, the population of the country probably will have halved. The government would be functionally non-existent, what vestiges are left have retreating into bunkers to preserve the records and artifacts, and to probably try and destroy nuclear weapons. Who would not retreat would be the military, who - though, like police, their numbers would have dwindled severely - and would be a major force, if not THE major force, in post-collapse America. However, they would have their hands full by the invading force of another country, who - faced with the most resource-heavy large nation-sized plot of land suddenly being virtually defenseless - would invade.

Assuming the world doesn't all suffer the same fate, by two years, California and the southwest would probably fall to Mexico, Hawaii to China, Alaska to Canada, Florida to Cuba or Venezuela (or possibly a hastily-formed alliance of Latin American states), and the northeast to a consortium of European nations under the pretense of restoring order and protecting lives. The Midwest, at least for a while, would become a de facto chain of several hundred independent states and counties. By three to five years, the US would be reestablished, but much smaller (probably 26 to 28 states) and centered on Chicago; the government would be very decentralized and probably with no national bank. It would never again become a superpower, the mantle of which would most likely go to China.

In short, it would be a massive regression with an enormous body count, but we have the advantage of having survived - and even thrived in - the 19th century, which is about the level of technology to which we'd return. Think less Road Warrior, more Dances with Wolves.