Calling up the militia as per Article I Section 8 is in effect a draft, and its necessity is explained by the opening lines of the 2nd, -- a militia being '-- necessary to the security of a free state, --".
Exactly - a point far too many people miss, and which took me a long time to figure out.
There is, however, one critical difference I'm getting increasingly torqued about.
Calling up the militia axiomatically presumes members thereof are already equipped and trained (whether by taxes or their own dime); they can serve, effectively, right now. A citizen has a duty, an interest, in being ready to defend self and nation at any moment.
A draft assumes participants are unequipped, untrained, and quite possibly unwilling. In many cases they have long been forbidden any suitable weapons, and will most likely not be allowed their own should they have any; they have no notion of where they will be assigned save "bullet sponge"; most will do what they're told, but that differs from enthusiasm. Conscripts may justifiably view a draft as "involuntary servitude" as they are treated as subjects compelled, not citizens serving.
Effective combat requires long training and familiarity with the arms used. Sure a conscript may get a few weeks' training with an M16 before combat insertion, but that's not enough to assure real confidence and effectiveness under stress. This country was founded on citizens who owned their own weapons, were familiar with them from youth, and thus had justifiable confidence in their abilities.
Ergo, the nuanced difference is:
- The Constitution allows for calling up any/all citizens* to defend the nation ... and guarantees the right of those citizens to arm & train themselves long before such service is needed.
- A draft, while on the whole the same as calling up the militia, presumes conscripts have neither arms nor training.
Personally, I've obtained certain arms & training on my own, so should I be "drafted" it would actually be a proper callup to service I am ready for.
A case could be made that: as the feds prohibit** ownership of ubiquitous military arms - the M16, M4 and such - in violation of the 2nd Amendment, then they have voided their power to call up the militia.
* - Yes, RP, I know you view that differently. Tough. The authors of the Constitution used the term "militia" for one purpose and "the people" for another, indicating they saw one as a subset of the other, and reserved RKBA for the larger set; also, 'tis oppressive to insist on reducing the current set of "citizens" to centuries-old limits, which so many have fought so hard to overcome.
** - Yes, if you are rich and/or connected you can buy a decades-old machinegun from a tiny and dwindling pool of used items subject to a 1200% (and growing) virtual tax. That is, for all practical purposes, a ban.