Countries with bitcoin standards don't just happen overnight. They all start with their own fiat currencies (or at least dollars in reserve) that they have more control over and start stacking corn on the down-low. Even gung-ho El Salvador didn't stack a bunch of BTC at once, they're buying 1 BTC a day or something for their reserves. Most countries will do so far slower.
Luckily for Bitcoiners, fiat isn't dying 1 country at a time... ALL fiat currencies are growing more weak simultaneously. That means that the world is giving up on fiat all at once and although some smaller fiats may bow out first, most will go into hyperinflation at the same time the dollar does. At that time ALL countries will be competing to get their hands on whatever bitcoins they can.
Therefore, a scenario where a healthy fiat currency is at war with a bitcoinized country has the exact same problem as a scenario where Dinosaurs fight a war against mankind... They simply won't exist at the same time.