Anybody who thinks that military defence can be assured by private businesses is delusional. That includes all the Mises Institute libertarians who follow Rothbard and Hoppe.

The keyword in the term Austrian economics is economics. It has nothing to do with defence or security. Even Ludwig von Mises, the namesake of the institute was clear that only the state can provide defence and security. For that it requires the monopoly of force suitably constrained by a vigilant population and enforced through effective institutions of governance.