I've seen a lot of authors contemplating the place and purpose of the militia in the modern era. I think I might have figured it out.
AmericanMercenary takes a look Popehat's http://www.popehat.com/2013/12/23/burn-the-fucking-system-to-the-ground/ and asks the question a few others have "And then what?", and proceeds to elaborate on the problems involved with losing the system entirely. Read the whole thing, then the other whole thing that gets linked to, and whatnot and so on and so forth.
Ultimately, it reminds me of something I saw a while ago, around the time of Egypt's uprising.
Essentially, when Mubarak's regime was falling, they turned loose all the prisoners as a "Hah, we'll show you!" and communities had to band together. To keep from completely degenerating into lawlessness, neighborhood watches and such had to be set up, and communities pretty much had to take over everything you'd normally expect to be taken care of by civilization.
That's the purpose of the Modern Militia.
Civilization is a precarious thing. It takes forever to erect, but can be toppled in very little time. Over and over through history, it has been seen that when society and civilization crumbles, it's a long time coming before it struggles back to its feet. The job of the militia is to keep civilization from falling due to the punch. It can rock and stagger from whatever blow may come, but if civilization keeps its feet, the odds are much better of coming through whatever transition about as painlessly as possible. If civilization hits the mat, it will likely be a long while before it regains its feet.
It's been said that something that can't continue forever, won't. When things break down, it won't be pretty, but it's absolutely imperative to keep things from devolving into complete lawlessness. Be active in your community, get to know your neighbors. Various disasters have shown that people can band together in times of duress. Wherever happens, wherever the telling blow comes from, it's the job of the militia to keep civilization standing, because the alternative is worse.
As a side note, the relative armament of the USA, combined with the ingenuity and production capacity, would allow the militia could well and truly ruin anyone mad enough to invade's day. but that's not really their most pressing job, with the state of the world and the military and everything else.