"then inexplicably vanished, leaving no trace behind in the historical records."
I had a better understanding of this when I had a chance to volunteer in Mongolia for a project. In the rural towns were large soviet style apartments standing empty. When the infrastructure changed, when the Soviets left, there was no one to pay the bills for electricity and heat. You CAN't live in an apartment without those. So everyone moved out to the country and lived in tents. Land was free and the new wealth and living opportunity were cattle and horses.
UlanBatar (sp?) the capital city had more access to international help so the system there kept going. Prior to my being in the capital the electricity had been on only 25-50% of the time.
If the international community had not stepped in to keep things going there was no choice for the residents except to move out to the country. The cities and towns would have been immediately abandoned.
What would happen in the US if the USDA and Bank system closed down tomorrow?
There is plenty of land, but not plenty of food except at certain times of the year. The FedGov would immediately go out of business. StateGov would remain, and LocalGov would be set up by whoever showed up to form a survival community. The People would make the best of the situation.
I assume the international community would either not respond or be in a similar situation, that is, the situation would be worldwide. Would it be Mad Max? Probably, just like the good old days in China except TribeGov would need a couple generations to get going again.
"then inexplicably vanished, leaving no trace behind in the historical records."
No trace, because the winners write the history books -- if they know what they're doing, that is.
Heck, most of the coastal Southeast would depopulate if air conditioning became unavailable. If I can't have air conditioning, I'm moving to a mountain top in Alberta.