If you look at history going back about 5000 years, and have a good understanding of political and military development, there is one constant truism: Consolidation and centralization brings power, and those areas that failed to do so were generally destroyed or consumed by those that did.
This. In the western world, where is our example of a libertarian state protected solely by militia? There might be some nations out there with a small military, but today they usually enjoy the protection offered them by stronger allies (via UN, NATO, what have you). The nations in Europe that did a poor job centralizing were quickly swallowed or overcome by the modern Westphalian nation states. It simply proved to be a more effective model at gaining, consolidating, and projecting power.
We must also remember that in the modern world a lot of these "militias" received foreign assistance against powerful invaders. Vietnam would not have been the same without SA-2s and MiG 21s and Afghanistan would not have been the same without the Stinger. All inventions of centralized militaries.
