Take the peer-to-peer traffic of the cryptocurrency network of any area, prevent it from getting outside that area (ie., just block it talking to IPs outside some range) -- then to that network, you can trivially control the total hashing power.
So split any cryptocurrency network into small segements, then add your machine to that network with a false history, design the network to be small enough, and your machine will out-hash the rest, and so it's history will win. Rinse-and-repeat.
It's trivial for any state to take down a cryptocurrency. There's nothing magic about it; it's an incredibly fragile system whose 'safety' relies on no one owning the network, and if that's the case, no one being able to russle-up huge amounts of electricity.
In both cases, this is false for states. So any state, if it wishes, can really do anything it likes.
The global bitcoin system is well within rearch of a hostile state 51%'ing it, even at the global level -- though the cost would be non-trivial. It would be trivial to do it in its own borders though.
As any state, of course will, if you can ever go into a shop and buy somehting with it. At that point you're imperilling a state's ability to use monetary policy to manage its economy, and that's an existential security threat. So bye bye your monopoly money tokens.