I anyone has the power to do this at the drop of a hat, it is China.
Drop of a bat is surely more topical for China?
I really can't see a nation mounting a 51% attack. It would cost a hell of a lot of money for a start, and I don't see what they'd gain.
If you want to shut down crypto within your own borders, then you can do this through legislation. It might not stop it entirely, but it would have a big enough effect to drive it so far underground that it's no longer a concern to the economy. Forcing exchanges to give out the details of their users for example, and then going after those users might be enough of a step by itself.
The only reason for a nation to mount a 51% attack would be to shut crypto down entirely, globally. Whether that would be effective or not is difficult to say. Whether such an attack would be mounted however is maybe easier. I don't think there's much chance. The reason being that although crypto may cause some fear and concern for governments, it also offers the allure of huge benefits through increased efficiency, reduced costs and indeed surveillance through the immutability of the blockchain. Nations may posture and bluster about crypto being a problem, but I think the chances are everyone will adopt it in the end... and I think this conclusion is lent weight by the fact that more and more countries are now investigating the potential of digital versions of their national currencies, even
the US is getting involved.