If you are at a war where your whole electric grid and internet and SMS are completely down, you have much bigger problems than currency not working.
It means your bank accounts are gone, your credit cards are gone.
And how are you actually going to use gold as currency? Walk into a shop and shave off some gold at the checkout? And how will the store owner authenticate that it's actually gold?
Bullets will be a better currency than gold in that scenario.
Trading jewelry for goods in desperate times is a thing. Even desperation in current times has examples of this. Pawn shops advertise in large letters that they buy gold.
But I agree, bullets are always a better currency; one of the positives (for people who responsibly enjoy firearms) living where I do is minimal restriction on such things.
I'm not sure war even needs to be that large for it to be a problem.
Basically unless its like China vs US then you don't need sell BTC to fund the country (even then I don't think you would; but if you never need to sell BTC its pointless).
If China vs US then realistically, China can and probably will cut a lot of the internet traffic between the two. Bitcoin only functions because of the public ledger; if Chinese miners aren't sync'd with non-Chinese miners then you have a double-spend problem and I can't see bitcoin not going to 0 pretty quickly.
Gold is similarly useless when it's heavy and hard to move. There is little chance of that, and Bitcoin offers advantages in ease of transfer, transparency of transactions, and lack of government interference.
It means your bank accounts are gone, your credit cards are gone.
And how are you actually going to use gold as currency? Walk into a shop and shave off some gold at the checkout? And how will the store owner authenticate that it's actually gold?
Bullets will be a better currency than gold in that scenario.