Russia has been paying for war with oil and gas revenue – taxation – not debt. This is beginning to fail, as Ukrainian drone strikes and sanctions have been cutting into tax revenues. But it's taken years for this to happen. Countries will be able to pay for war just fine on a Bitcoin standard.

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But I certain point could be reached, where the country (Russia or any other) wouldn’t have the necessary resources the keep going. That was the whole point of going of the gold standard.
Countries run out of money fighting wars regardless of the currency. Fiat isn't magic in that regard; inflation is just a other form of taxation.