>Serious question, if the United States decided to unilaterally cut North Korea off from the Internet, how hard would that be to do? Could we just knock out a few cables?
You will have to cut cables going to Russia (and\or out of Russia) and China at least.
The Kims are likely the only reliable bet for people who would actually survive a nuclear war and the following nuclear winters, if the book Nuclear War by Anne Jacobsen is to be believed. They're the only ones that have built their bunkers deep enough (under mountains) that modern thermo nuclear weapons wouldn't destroy them, and they have years of supplies down there.
You will have to cut cables going to Russia (and\or out of Russia) and China at least.
Not to mention wireless comms.