Bitcoin and other crypto require a well functioning internet infrastructure, otherwise it's just data on a hard-drive. Disconnect a country from the Internet (or filter the traffic) and you're forking the chain. Spend on both networks by buying other (unfiltered) crypto. There are states (even large ISPs) that can do that. Also, the first thing to go in case of global conflict is the global Internet. Countries will t…
Pedantic nitpick - there are crytpocurrencies which are not distributed proof-of-work blockchains, and would not necessarily lose all of their value.