Bitcoin relies on the internet, and the power of computers around the world to power the transactions and iner-workings of the BTC. If the internet "goes down", there will be nothing left to power the whole BTC process, causing it to breakdown.
internet is available in certain locations of the world, you wouldn't be thinking about bitcoin if there was a nuclear war going on. You would just stop all the trading and whatever transactions you could make with internet. Once something happens that depends on life or death, money and bitcoin doesn't mean anything anymore.