All they'd really have to do is announce that they were going to build enough computing power to initiate a 51 percent attack on bitcoin. I imagine the NSA is already there but it would be a nice cash injection for Intel or whatever.
Bitcoin has a very, very short history of being a decent store of value and the US Government publicly saying they plan to break it would collapse the entire scheme.
It’d have to be a very prolonged 51% attack. Other nation states would likely join in, and it’s not like the USG can stop millions of people around the world from mining. Even if they succeeded in driving the price of most coins to zero, it’d be a Pyrrhic victory with USG looking like giant, insecure assholes spending a huge amount of resources just to destroy something that democratized money and banking. Months later, a new white paper would come out that finds a way to disadvantage state actors and viola, the game of whack-a-mole begins anew. People want out of dollars because the US economy is mismanaged, and that demand isn’t going away.
Bitcoin has a very, very short history of being a decent store of value and the US Government publicly saying they plan to break it would collapse the entire scheme.