Would this not be solved by insurance?  If you live in Florida, your house insurance is probably higher due to hurricane risks.  When a hurricane hits, the insurance covers damages or goes bankrupt.  Not really much different now except that the government may kick in help, so you'd lose that safety net.  But hopefully, you have insurance & choose a reputable company.

Same could be done for defense.  You could purchase defense insurance.  It may be more expensive if you live near the border.  This insurance would pay for a patrolling submarine or F18.  Maybe setup SAM sites.  If there's a threat to insured persons/property, defense mechanisms could be activated.  Competing defense insurance companies could even collaborate to mark friendly aircraft to prevent friendly fire.  The rainy day fund could pay for more defense or to pay for damages.  If it went bankrupt or never had insurance, you could pay dues to a private army to protect you.  Lots of options.  Even competing options that could work together.

In general, AnCap removes a lot of those non-natural threats.  Not enough people willing to pay large sums for large armies when they can purchase nice cars, TVs, food, houses, etc.  Unfortunately w/ fiat currencies, we don't really get that choice right now.  And our countries can pretty much start fights w/ anyone without our consent.