"There are two thought's on that. One that the "militia", a citizens military, is an important defense of the country. At any time, if an officer of the law feels it to be necessary, they may call upon and "deputise" citizens to support them. That would include invasion. The second thought is that the men who created the US government did not trust governments. An armed citisenry could be considered a deterent from tyrany. A concept that is easy to believe since historically one of the first things that happens when a government is being corupted and overcome from within is that those taking power remove the right of common citisens to own or use weapons. (Hitler did it in Germany and it was a tactic used for centeries in many parts of Asia and the Orient, for just a few examples.)"