Suppose that a country decides to separate completely from the outside world, as such. Do not take any leaks. If I understand correctly, once local miners have processed all international transactions in each other's program management tools, they will only have local transactions, blocks, nodes and miners left. A (long) while later the difficulty fits down, and you now have a local fork of BTC with a lower POW and no code, even if it's the same code. Travelers can go there and spend their balance before use, either by sending it to another address they own, or by sending it to the local population, although this is clearly separated from all transactions that they carried out abroad.
Assuming they only keep up with the code base and do it for a long time, say a year or more, what happens once connectivity is restored? Is their local channel completely destroyed because of the reduction in the amount of prisoners of war? Would local nodes be forced to go back to the fork time and download international blocks? All these transactions valid at the time, gone? And the roles are reversed: if this country had enough mining power to have more prisoners of war?
And as a bonus, how do Layer 2 solutions, assuming they are the only traffic that can cross the border all the time, affect this scenario? Could you load funds on the Lightning network from both networks and effectively double your balance by removing it from the outside network?