I disagree with the concept of drafting people...basically gets lots of productive people out of what they are doing and into combat training.
However, I would "encourage" (and how a government can encourage without use of incentives [aka tax-funded bribes] or propaganda [paid for by ads?]) the practice of proficiency with smallarms and RPGs. A small professional army may be possible, though paying for it would be the sticky point.
My guess is that having most households well-armed (which would happen based on the requirements for citizenship and imigratin in general) would do as a deterrent. Since no taxes or bad laws would hinder production and purchases of arms, I'd expect that they would be relatively inexpensive.
My hope would be to reduce the "govt" to a PR agency. It would not even be able to enforce immigration limits for lack of coast guard or any other alphabet soup agency...would just hope that the peculiar character of the country would keep the riff-raff looking for outright hand-outs away [since there would be no social services OR any call for pro politicians]. The only question is how to select the "govt PR staff" and I would just leave that up to whoever wants to deal with the issue as a hobby...
Dream on, right?
If you believe in freedom and means of protecting it...you might be a gun nut.