Agree with
@Bacle, the type of disaster is going to have a huge impact.
Going off things I think could reasonably Mad Max the US (So not stuff like zombies that can safely be stored in the fantasy drawer) The Yellowstone Supervolcano blowing up and taking North America with it seems most likely, followed by civil war.
Assuming the mildest effects of the supervolcano, we can look forward to the worse sequel to
Eighteen Hundred and Froze to Death. Note that Tambora threw 12 cubic miles of ash into the atmosphere, and it resulted in basically two years of Game of Thrones style endless winter, there was so much ash in the atmosphere that the ground never warmed and spring never came that year. Abnormally harsh and violent storms resulted from the bizarre disruption to warming patterns, most of the world's crops and livestock died, everybody worldwide froze and starved.
That was from 12 cubic miles of ash. The last time Yellowstone felt like stretching its legs (640,000 years ago), it cut loose with
250 cubic miles of ash. This was
not the largest eruption Yellowstone's ever had. That caldera is an absolute monster. It's quite likely it will be weeks to months before humans who aren't on a space station
see the sun again, much less the years it will take for there to be summer again.
American society is going to be based entirely on coastal cities because Yellowstone already killed everybody in between the Rockies and the Appalachians. The San Fernando Valley will briefly have most of the US's supply of food. Fishing is going to be the biggest source of food. Yes, the oceans will be suffering too, and the horror-weather a supervolcano spawns means it will make
Deadliest Catch look like Disneyland, but there's a
lot of ocean and the entire US fishing fleet is now only serving the west and east coasts + Alaska and Hawaii. Even at dramatically reduced catch rates, this will be the major food source during the several years that no crops will grow.
I expect the east and west coasts to eventually create their own separate governments. The biggest Mad Max style wars will not be cars fighting for oil, but be over the few fishing grounds that still produce. Currently, a Chinese fleet showing up and scraping the fishery dry is an economic hardship. Then? Each of those ships is a town of your people who are going to starve to death. Navies will adopt a shoot-first attitude towards poachers on their precious fishing grounds.
Hawaii will likely split off from the rest of the US but its Naval assets are likely to mostly travel to the West Coast to protect the continent. Carriers will be less useful than they have been historically, because of the superstorms making flight deadly. However, they're still the 800 pound gorillas in the seas and the Disunited States will use regular air patrols to sweep for enemy fishing boats as long as they have fuel. Given that the weather will be freezing cold everywhere, most of Canada's surviving population will migrate south into the states seeking refuge. The West United States may try to annex Baja California to get at warmer climates and fisheries.
When materiel starts getting scarce, expect military groups to cross the two mountain ranges and start seeking out depots buried in ash to loot for ammunition, fuel, rations, etc. This will eventually lead to clashes between the two coastal States over who owns which depot. Expect a few battles.
In the end, when Summer finally returns after several years of continuous winter, US culture will have evolved dramatically.
Fish will be the biggest part of the diet, and by this point fishing will be part of the national identity, the way cattle are now. Fishermen will be revered as the toughest SOBs on the planet and entertainment will celebrate iron-hard fishermen who fight pirates, stop invasions, and save lives the way cowboys fighting rustlers were revered in the 60s and 70s.
The military will be seen as heroes due to both being able to bring in vast quantities of life-saving supplies from depots and fighting off fish-pirate fleets.
The states will regard themselves as naval powers first and foremost and the other military branches will be decidedly inferior.
International trade will be long gone, most fleets will be too busy protecting fishing grounds and will shoot at any suspicious ship they see. It's likely to be many, many years before this paranoia fades and people start to ship goods again.
Once summer returns, groups beyond just the military will start leaving the coasts and moving into the heartlands to reclaim the earth. To be sure, some rugged types will already have done that and put together cabins and holdings, but there will be a new land rush as hasn't been seen since the old west. Both coasts will move towards the middle. The recolonization process will likely take half a century or more and be a second wild west/mad max era. Depending on how serious the clashes are, either the two will wind up becoming East and West United States, or they'll eventually reunite. The heartlands will have, by this time, absorbed insane amounts of minerals from the mountains of ash dumped into them and food will suddenly become amazingly abundant, this will last for decades to centuries.
The Mississippi will likely have become more wild during the time it was unattended. People have spent a lot of time easing it, removing sandbars, dredging out rapids, etc. Several years of super-wild weather is likely to fill it up with snags and sandbars again and it will need some care, but will again become a major part of shipping and quite likely if the Disunited States never combine again, it will become the border between the East and West United States.
Against all odds, somehow Oklahoma will again be the last place colonized just like it was last time, when settlers went around it to colonize the rest of the continent.