Sovereign currency isn't really backed by military force so much as it's backed by trillions of $currency being owed in taxes or repayments over a given time period, meaning that obtaining currency has value to individuals and companies with outstanding debt and they're willing to swap goods and services for it. The military force is useful only in the narrow sense that being attacked by neighbours tends to damage the ability to perform services and repay debts and collect taxes, and many nations have perfectly adequate currencies but limited ability to project military power.

(And FWIW cryptocurrencies now have some debt repayable in them which helps stabilise their price somewhat, but the markets are relatively thin and short term, and the companies taking on crypto repayment obligations are generally trying to obtain the crypto by swapping for other volatile tokens in a zero-sum bet rather than borrowing to produce real world goods and services to trade or securing their debts against valuable real estate)