I try to use the “Rules of 3” [they are NOT engraved into stone, but real good guidelines] 3min with out O2,3 hours without good warm shelter,3 days without water,3weeks without food & you get death.
Now the average store has about 3 days worth of food stored in it. so you can roughly figure 1 week before the food riots start, but when those in the inner cities have figured out that there is NO law & order avail, they are going to tear themselves apart right away, and the survivors will start making their way out of the cities towards the suburbs. And those in those areas will hold out for a bit, but won’t last long. So then they all will head to the rural areas. I’m just hopin that the Veterans that are left will start to band together & put the world back in some sort of a working condition/s.
It takes a continent level of catastrophe to result in what you are suggesting.
We have seen a few cases historically. Most involve war, espcially recently.
A long term crash of the power grid in the U.S. could trigger it, especially if it happened in winter.
I suppose a widespread nuclear strike might do it.
Virtually the entire rest of the world is in worse shape to survive such an event than we are.
Could happen, but the chances seem small.