As has already been mentioned, some states, including Virginia, do have such a thing, which I believe is called the state guard or something. It's not exactly like the old British Home Guard but the idea's the same. They're theoretically supposed to guard important things when necessary, basically relieving that responsibility from other military units. There must not be much of a need for them, however, since there doesn't seem to be any form of recruiting going on. If there is, it missed me.
However a good the idea might be, I think a basic problem is rather that few people would want to join, rather than lots. Recruiting is always a problem for the armed forces, both here and elsewhere, especially when there has been a tradition of voluntary military service. That's not only true in the United States but in some other countries as well.
Another basic problem of such an body is that it competes with other elements of the military for recruiting. Some elements of the population would have no interest at all in joining and ultimately, I suspect recruits might be drawn from a relatively small base, which is not necessarily bad, only that the pool of prospects is not unlimited. Of course, that's true of the military in general. You quickly realize that you need young men of a certain age group for your army and at any given time, there are only so many and even some of them wouldn't qualify under the most ordinary of standards. Like it or not, a unit of old men is of limited usefulness (but not quite useless).
Another problem with such units, including old-time militia units in some places, was that the active military sometimes tended to view them as a source of trained men, thereby robbing them of (probably) their best members. Naturally, this presupposes there is a real need for military units in the first place.
Shoot low, sheriff. They're riding Shetlands!
Underneath the starry flag, civilize 'em with a Krag,
and return us to our own beloved homes!
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