There are many people who somehow believe that crypto-currencies offer protection from state interference. In American history, one example would be the compulsory exchange of gold for dollars, which was enforced in 1933*. Those who failed to comply faced possible fines, or even jail sentences. Anyone who still thinks that crypto-currencies can’t be confiscated face disappointment – in fact it’s even easier to do so than seizing gold. All you need is to reach agreement with Apple, Microsoft, and Google – to copy all the keys they have sitting in memory.
Things are slightly harder with Multisig – but it’s still possible. You can ‘agree’ with hardware wallets, or simply infect them with viruses. For the rest, simply bear in mind that all internet communications are monitored, and it would be no problem at all to halt all transactions. The entire procedure could take up to a month, and yes there’d be a lot of jangle in the media, of course – but nothing that couldn’t be tolerated in the interests of national security.
* by the Executive Order of President Franklin D Roosevelt.