Although the draft as we know it today in the United States got its start more or less during the American Civil War, it has a long history in association with militias beforehand.

The colonial military establishment was a fairly ad-hoc system, which was based, in part, on English legal customs, but was also organized in response to the threats that had to be dealt with. English legal custom stressed the virtuous citizen-soldier as the best possible defense of a free society, owing to a great deal of legal and political thought at the time. To boil it down to its constituent parts, it was based on:

1. volunteerism made for better, more motivated fighters, which was owed to
2. community "interest" in the form of propertied men defending their property (and its somewhat more complicated and eventually neglected belief that the more property, the higher the communal debt, and the higher responsibility to bear arms in the community's service)
3. distrust of soldiery, military institutions, and permanent standing armies: soldiers were by definition "disinterested" and could very easily be put to the service of a corrupt politician

So militias, generally, were intended to be made up of propertied men with an inherent interest in protecting their property, who would fight when called, and then disband so there was no more threat to the society which needed armed defense.

There were a number of functional problems with this system, which led to the general complexity of colonial, and eventually US, military systems. The first and most egregious was that, often (believe it or not) those men with a great deal of property were seldom interested in serving to protect it. This led to a substitution system, whereby those men would furnish a replacement to serve in their stead, or pay a fine.

Cracks in the virtuous-citizen ideal aside, there were other issues to deal with: farming depends on specific timing, and having all of your farmers serving for months away from home means untended fields and economic distress for families and communities. The solution there was very short-term enlistments for militia and an increasing reliance on volunteers to serve longer terms of service.

Volunteers were, again, expected to be those men with money and time enough that they could be spared from the community and make little economic impact in their absence. In practice, it meant men of no means whatsoever volunteering in the face of few alternative options.

Still, it more or less got the job done in terms of frontier warfare. During large-scale conflicts, on the other hand, it didn't work out all that well. During the French and Indian War and the War of Independence, the bulk of the fighting was done by professional soldiery: training and discipline made a much more measurable impact than zeal and idealistic civic virtue.

In the War of Independence and the War of 1812, militias were used both as a mean of temporary defense and as a recruiting farm for volunteers, but volunteer quotas (especially for the latter war) were often not met, and so the militia was used as a draft system: men were basically picked right out of militia companies and drafted into the regulars. In practice, this was used as a means of soft coercion more so than it was done: "we better get all of the volunteers we need or we'll draft you!"

You can still see problems with this system: it is based to a large extent on the popularity of the war. Paper-strength meant little if you couldn't get your men organized and on the field when you needed to.

And so, during the War of 1812 and after it, alternatives were sought. They needed a system that essentially took away the rights of men to refuse service, either directly (by acts of collective indiscipline during the War of 1812 among militia units), or indirectly, by refusing to muster, deserting before the service time was up, or by the conflicting militia laws of various states.

A letter from Henry Dearborn to Governor Simon Snyder of Pennsylvania written in August, 1812 typifies the difficulty of dealing with civilian resistance to military measures. Dearborn writes in a drippingly polite tone throughout the letter, which is stated as a request, rather than a legal call to arms in times of war. Toward the end, he stresses:

"it is highly desirable that there should be an understanding amongst the officers and soldiers before they march, that it will be expected they will make no objection, if required to cross over to Upper Canada, for without such a condition there would be little, or no use in their marching to Niagara."

The authorities had to ask and men had to agree to serve under these conditions. The standing force was too small and volunteers too scarce to effectively fight without a back-and-forth understanding not only between federal and state authorities, but also between state and individual.

It took the Civil War for a conflict to expose these weaknesses to such an extent that they needed to be dealt with directly. Somewhat ironically, it was the Confederate states that first instituted a coercive draft in 1862, calling for all white men between the ages of 18 and 35 were liable to serve for a term of three years. The Union followed roughly a year later on similar terms, but still married everything to militia service. Men were placed in local militia units and subject to drafting thereafter.

Coercive drafting, of course, was still used as a stick: if enough men would step up and volunteer, the draft would be unnecessary. The carrot came in the form of enlistment bonuses and other favorable benefits for volunteers.

To get to the modern idea of the draft, the idea of the militia as a component of American social makeup had to be done away with; this was already in full swing by the Civil War.

Perhaps someone else can pick up after the American Civil War; I can only go on in very broad generalities. If you have any more questions related to the militia system I'd be happy to help.

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Early militia organization and history are in a number of books, but three that might be most helpful are Lawrence Cress, Citizens in Arms; Saul Cornell, A Well-Regulated Militia; and Richard Kohn, Eagle and Sword.

Problems with the militia are often referenced in general histories of the war of 1812, but Edward Skeen's Citizen Soldiers in the War of 1812 is the most useful in that regard.

The quote from Dearborn is part of personal research to the NARA in Chicago I conducted last summer, but it can be found in M6: Letters Sent by the Secretary of War Relating to Military Affairs, 1800-1889