To provide for organizing, arming, and disciplining, the Militia, and for governing such Part of them as may be employed in the Service of the United States, reserving to the States respectively, the Appointment of the Officers, and the Authority of training the Militia according to the discipline prescribed by Congress;
Got that, if you aren't organised according to Article 1, Section 8, Clause 16, you AREN'T the militia!
The Sedentary, reserve, inactive, unorganised, general (or other term indicating INACTIVITY) Militia, has always been unorganized and untrained
Active Militias, that is THE organised, enrolled, embodied, active (or other term signifying active) Militia, can be supplemented if necessary by the ballot (selection by lot)--in other words drafted from the Unorganised militia draft pool.
The term "unorganized" did not begin to emerge until the 1830s and 1840s, when a massive wave of opposition destroyed the compulsory militia system. Nobody wanted to serve in the militia. State governors and legislators wanted to be able to accommodate this desire, but they were bound by the 1792 Uniform Militia Act, which stated that every white male aged 18-45 would be in the militia.
Militia service was so unpopular that Delaware abolished its militia system altogether in 1831. Massachusetts eliminated compulsory service in 1840, followed by Maine, Ohio, Vermont in 1844, Connecticut and New York in 1846, Missouri in 1847, and New Hampshire in 1851. Indiana classified its militia according to age in 1840, and exempted all but the young men from service. New Jersey withdrew the right to imprison a man for failure to pay a militia fine in 1844; Iowa did the same in 1846, Michigan in 1850, and California in 1856." - Mahon, John K, The History of the Militia and the National Guard, p. 83
Not a good situation if you were a supporter of the militia system (as opposed to a professional military).
However, the 1792 Uniform Militia Act explicitly allowed the states to determine who was exempt from militia service. So states divided their militias into two sections, the "organized" militia and the "unorganized" militia. In this way, the letter, though not the spirit, of the 1792 law could be complied with. However, only the "organized" militia would have responsibilities. These people would be volunteers, people who actually wanted to perform militia service; they gradually evolved into the National Guard. These people would have uniforms, guns, and would drill, review and encamp.
The other people were the people who did NOT want to be in the militia. Accordingly, members of the "unorganized" militia were NOT supposed to perform any duty or carry any weapons or have any responsibilities. All that would remain was the nominal authority of the state over them for military manpower purposes. This group of people had no militia responsibilities at all (in some areas they had to register, like for the draft today). In this way states could flaunt the spirit of the 1792 Uniform Militia Act, while nominally keeping to the letter of it.
The term "unorganized militia" was kept in use in subsequent decades as a statutory "reminder" that the state could still obligate its citizens to perform military duty, should it ever want them to. Eventually, U.S. law in the early twentieth century picked up this same usage for the same reason: by creating the "unorganized militia," the United States could guarantee usage of this manpower for military purposes, should the (remote) need ever arise.
Joseph Story noted the dislike for militia discipline in his 1833 Commentaries on the Constitution (3:§ 1890)
And yet, though this truth would seem so clear, and the importance of a well regulated militia would seem so undeniable, it cannot be disguised, that among the American people there is a growing indifference to any system of militia discipline, and a strong disposition, from a sense of its burthens, to be rid of all regulations. How it is practicable to keep the people duly armed without some organization, it is difficult to see. There is certainly no small danger, that indifference may lead to disgust, and disgust to contempt; and thus gradually undermine all the protection intended by this clause of our national bill of rights.
REPEAT TO REINFORCE THE MESSAGE: a category was created to keep the letter of the law, but not its spirit--The Unorganised, Sedentary, reserve, etcetera militia.
So, being in the "unorganized militia" conveys to you no rights, only the possibility of responsibilities. All it means is that you belong to that class of the militia which has no responsibilities. Being in the unorganized militia allows you to do not a single thing, because only the state and federal governments can create (working together) active militia systems. To date, their interest in doing so has largely concentrated on the National Guard.
Again, let me emphasize that there is not a single right guaranteed to you by virtue of your being in the unorganized militia.
The militia system intended by the framers is utterly defunct nowadays, the Pseudo Second Amendment "militia" advocates have seized upon the phrase "unorganized militia" in the US regulations. The subtle rhetoric trick here is to claim the "unorganized militia", (a term simply meaning eligible citizens) is the same as the "organized militia" (a term meaning amateur army) - EXCEPT when it comes to any State and Federal controls. They thus try to have it both ways, all the good things about the term (military connotations), without any of the restraints implied (government authority). However, it's very much an invention without any basis in fact. They just hope no-one in the audience knows enough history to call them on it, and they're often right.
But the propaganda here has it exactly backwards. The whole "unorganized militia" aspect was a much later legislative maneuver for people to GET OUT of the real (i.e. "organized") militia, akin to say getting out of the draft by being shuffled into a "reserve draft" category. It was for people to escape the conscription-like service requirements, not a license for private paramilitary groups. The structural details of the militia system were concerned with the extremely difficult task of funding and running an effective military without having a large standing army, and had nothing to do with individual gun rights. "Unorganized militia" in modern terms was more a draft-dodging loophole, not a Rambo clause.
It's something like if during the Vietnam War, people could get out of the draft by merely going into the "unorganized draft", which was supposed to come forth if the US was invaded by Vietnam. Formally, if you read that many decades later, you might naively think it actually implied some military service, whereas knowing the historical background gives it a very different aspect.
This whole "unorganized militia" banner is a bit like people calling themselves "draft dodgers", and then claiming veteran's preference because they've been part of a "dodged draft". The word simply means the opposite of what they think it means.
The whole point of creating an unorganized militia was so that the majority of citizens would have no militia responsibilities at all. This is not a big secret that I somehow uncovered; it has been well known among military historians for ages.
For a good reference see: MILITIA - HISTORY AND LAW FAQ
Some standard works on the militia and the American military are:
Cress, Lawrence Delbert Cress. Citizens in Arms: The Army and the Militia in American Society to the War of 1812
Cunliffe, Marcus, Soldiers and Civilians: The Martial Spirit in America, 1775-1865
Mahon, John K, The History of the Militia and the National Guard
Millett, Allan R. & Maslowski, Peter, For The Common Defense: A Military History of the United States of American: Revised Edition
Riker, William H, Soldiers of the States
One of the few Law Review articles discussing the historical militia is "The Militia Clause of the Constitution" by Frederick Wiener 54 Harvard Law Review 181(1940).
See also Adam Smith's Wealth of Nations, Book V, Chapter I (Of the Expences of the Sovereign or Commonwealth), PART I: 16-27 (Of the Expence of Defence) for a critique of the miltia system from 1775.
Also, David Chandler & Ian Beckett, The Oxford Illustrated History of the British Army (ISBN: 978-0198691785) has a section on the Amateur Military Tradition (I.E., the Militia).