LONG history lesson follows:
England and by extension the American colonies had a deep and abiding suspicion of standing armies, and any look at history shows why: historically, standing armies tend to either support tyrants, take over the government, or become a political force that makes sure they always have wars to fight. So deep a suspicion that in the late eighteenth century both England and the newly formed USA had laws forbidding military spending more than a year or two in advance. Both proclaimed at least as an ideal that a country that is governed by a representative legislature and champions individual rights should be defended as much as possible by citizens, not by an elite of semi-mercenary professionals. However maintaining some minimal core of professional soldiers seemed indispensable.
Against the objections of many anti-Federalists, the US Constitution provided for the establishment of a Federal Army and Navy; and Article One Section Ten forbids the states from having their own foreign policies, independent treaties or their own standing armies or navies. Foreign relations, and the means to conduct wars with other countries, was to be the monopoly of the Federal government.
So what were the states allowed? They were allowed militias, the ideal of the armed citizen who could be summoned in an emergency to the common defense; a democratic tradition going back to the citizen-soldiers of democratic Athens, the Militia of the Roman Republic, and the more recent example of the Swiss Confederation. Composed of mustered citizens who would presumably be unwilling to abandon their civilian lives indefinitely, a militia would be ideal for self-defense while poorly suited to foreign adventurism or the imposition of dictatorship.
Since the militias organized by the several states might have to work together to repel an invasion (not an unthinkable occurrence in the early days of the Republic), the Constitution also granted the Federal government joint authority with the states over the militias- also bitterly objected to by the anti-Federalists, and answered by Hamilton and Madison in the Federalist #29 (which incidentally uses the term “militia” in a context that makes it clear that they’re talking about every able-bodied man who could carry a gun). The Constitution allows for the Federal government to enact uniform standards for the militias, and the first Militia Act of 1792 specified the standard armament all men were to be required to own.
However, unlike the Swiss example, the United States has never in its entire history had a system of universal military training- in fact such a system would have been regarded with suspicion as an attempt by the Federal government to regiment society. In the Federalist #29 Hamilton insisted that it was neither necessary or practical for the Federal government to enact such a scheme, and the standard was that a core of regular volunteers- a “select” militia- would be the states’ go-to guys with the rest of the male population merely to present themselves with arms once a year or so to demonstrate their potential readiness.
In the ensuing decades, while armed citizens and volunteers were important fighting Indians on the frontier or in disputes on the northern and southern borders, in much of the country the custom of mustering the male populace to arms became moribund. Hastily summoned untrained civilians had performed poorly during the British invasion of 1812, and afterwards virtually no one expected that such forces would be of any military use. Contemporary cartoons mocked the musters as farces and Federal oversight was allowed to lapse. During the American Civil War the Confederacy enacted a flat-out draft, while the Union paid millions of dollars in signing bonuses to get as many men as possible to sign up for a three-year tour of duty in the regular Army. Units were still organized along local lines and at least initially were led by elected officers, but it became clear that to be militarily effective a citizen army had to be trained, armed and provisioned by a national government.
After the Civil War the trend of relying on trained professionals or semi-professionals intensified. Large cities developed police forces, and the states came to rely entirely on select militias composed of a limited number of recruited volunteers, that other than not being full-time employees of the state were troops in everything but name. Reaction against immigrants, leftist radicals, striking workers and the poor led to a suspicion of armed populism and to increased government control over the franchise of bearing arms. In 1886 the US Supreme Court ruled against Herman Presser’s appeal of his conviction under an Illinois law that forbade anyone but the limited number of approved state Guards from drilling or parading as organized military units. Presser tried to raise several Constitutional issues regarding the Second Amendment and Article One Section Ten, but the court focused on the narrow technical legality of his conviction and reiterated its opposition to incorporation of the Bill of Rights under the 14th Amendment’s due process clause.
The Spanish-American War was fought by a combination of regular US Army troops and volunteers from the state militias. Outdated equipment and lack of standardization limited the effectiveness of the volunteers and lent impetus to the passage of the 1903 Militia Act (otherwise known as the Dick Act). It continued the tradition of regarding all adult male citizens as at least potential militia members (called the “unorganized militia”) while making the “organized” state militias officially Army Reservists. To all intents and purposes today they are effectively volunteer Army troops, save only that the state governments retain some joint authority over them.
Someone mentioned the State Defense Forces? Well as mentioned earlier, the third clause of Article One Section of the US constitution forbids the states from keeping troops but with the proviso “without the Consent of Congress”. In the federal code 32 U.S.C. § 109 the states are given explicit permission from Congress to form state-only defense forces. The State Defense Forces are essentially the states’ own armies, and in theory the Federal government could at any time revoke it’s permission and the states could be forced to disband them.