Libertarians and National Defense

 Libertarians think that national defense will not be needed in a Libertarian society because a Libertarian society would not care what other nations do (This in itself makes no sense. Of course the Libertarian nation would care if it's citizens were taken hostage or if other countries placed extortionate tariffs on its products and services.). This is a very dangerous view to have. If we have no military, then what is stopping anyone, even just bandits or a businesses private army, from conquering the United States. On top of all this, remember police and courts would be privately owned, so we would go back to the medieval ages, with individual barons and nobles squabbling over land and power, and the innocent villagers caught in the middle.