What? Militias aren't some Libertarian fantasy force. Militias are what countries with limited resources used in lieu of a standing military. They're also all but obsolete in a world where military technology has advanced to the point that private citizens can't be expected to field their own effective arms
Uh, not quite. Even some of the best and wealthiest armies of the times used militias(English Empire, Prussia, etc). The idea was more to have a centralized, smaller, professional fighting force that, when called upon to fight in or defend an area could count on members of the local population to join the ranks and fight. This reduced the number of troops that needed to be trained, fed, housed, and paid, but still allowed a state to field a sufficiently large defensive force. Their record in offensive combat is atrocious, however in defensive situations militia can play a strong role, especially in long-term, low-intensity conflicts. They can be used in anything from guerrilla actions, to urban combat, or even in COIN operations alongside regular troops (my Master's thesis was about this). In urban combat-or at least, urban combat that seeks to do anything more than just simply leveling an entire city- military capabilities change drastically. Armored vehicles cannot safely navigate tight streets and air support is severely restricted if the city has a significant skyline. Soldiers still have to clear individual buildings, leaving them vulnerable even to arms available to (American)civilians such as shotguns and semiautomatic rifles. In places where fully automatic weapons are available to civilians militia have shown to be very effective against regular troops. But in any case, the definition of a militia since at least the 1700s was civilians who arm themselves with their own weapons and join the lines of regular troops. As carefully as the writers of the Constitution chose words, if they had meant anything else they would have chosen a word other than "militia".
And the fact is, if open conflict comes to American soil, militias (in function if not in name) would have to be utilized in some fashion due simply to the size of the country in terms of both landmass and population. Here, I think, is where your "libertarian fantasy force" comes into play, as I do not see large scale open conflict occurring on American soil as long as America remains a cohesive entity.