If a party could control the irregular tide of Bitcoin’s value rising and crashing, then it holds a means to stimulate or impoverish whatever economies rely on Bitcoin. This could be particularly attractive for parties who want to avoid formally declaring sanctions or for situations where sanctions might not be effective.

I would see a 51% attack as a nuclear option, since it could take a while for cryptocurrency users to once again believe that the network is secure, but you could imagine a situation where there is geopolitical conflict with a country, the people in that country adopt a cryptocurrency due to fiat currency collapse, and then it becomes useful to induce a cryptocurrency collapse as well.

Edit: oops, might have added this comment in the wrong place.