You assume that Americans are capable of reaching the border in the first place. No doubt they’ll starve to death while cursing their enemy political party first.
Come to think of it, what happens if supply lines break down and fuel stops being delivered? I could imagine the mother of all famines, as food won’t make it into the (food) desert and people can’t come out.
You know what happens. Absolute fucking chaos followed by militias and what’s left of the US government tearing up the US like puzzle pieces. Millions will die if they don’t commit suicide first. Keep as much food as you like. Store as many guns as you can get. It won’t matter. It will become the supplies of the person who steals it from you.