It is backed by force. How do you think you collect debts when people don't want to pay?
The military is useful to ensure property in general is still a thing (including ensuring people aren't rounded up and tortured by enemy soldiers hunting their crypto keys) but doesn't play much role in ensuring that people sell their labour to meet mortgage repayments.
Debt repayment is enforced by exactly the same rule of law and exactly the same law enforcement as the property ownership of anyone else, including cryptofantasists.
Tax collection is backed by force.
Try not paying your taxes and see what happens