For example, if the rest of the world adopts BTC as the world's reserve currency, the USA would HAVE TO purchase crazy amounts of BTC, and defend against a 51% attack by investing into mining.
Meanwhile, the USA could try to preempt this future, but only the USA is incentivized to have USD be the world's reserve currency. As such the USA would need to threaten every other country in the world to stop using BTC, but its not clear if that causes BTC adoption to drop, or the Streisand Effect to kick in.
And ofcourse, "mutually assured destruction" stops any large country from employing too large a threat against any other large country.