> especially if you have super strict civilian firearm laws like Costa Rica that make putting together defensive militias much, much harder.

Not refuting the larger point, but this part sounds questionable. For a country, rifles are easy to provision: just buy enough and keep them in well-guarded armories in strategic locations. Trained soldiers are much harder to come by.

South Korea has relatively strict gun laws and very few people own one at home. But almost every male goes through two years of military service. In case of a foreign invasion, it won't take 24 hours to summon millions of civilians, just throw each one a rifle, and there's your army.

But no one else has a standing army in this scenario so there is no such modern military that can conquer you before you can remilitarize. The moment one country starts militarizing in a world without militaries, you know their intentions and can start remilitarizing to counter.

But let's say Costa Rica had a standing army - are they going to stop that modern military from conquering them? Probably not, so why waste money on a half assed defense?