You don't need army to defend the math.
You might need one to defend yourself in the meatspace. And you can pay it with Bitcoin.
Discussion
Interesting. So yes a private army at your private property
Would be tough to attack. A deterrent as well
I'm thinking about a coop of ex Navy seal mercenaries.
But even with Elon Musk resources, you're too easy of a target if you become too uncomfortable for the state.
That's why Satoshi chose to fight the state on a different plane. Building a parallel society with its own rules. Peaceful opt out.