One alternative, I suppose, would be to step up existing programs that allow foreign nationals to move to the front of the line for green cards and eventual citizenship in exchange for doing their hitches with Uncle Sam.This brings up a semi-unrelated point. I'm not quite sure where I stand on the multiculturalism vs. assimilationism debate when it comes to immigrants. But it does seem that a strongly multicultural America would undergo a distinct weakening of national pride and patriotism. That, coupled with a draft, just might create the sort of 'Roman barbarian soldier' scenario described above.
Rome went a similar route after it ran into its own military manpower problems. Soldiering became the most reliable ticket to citizenship -- that is, until citizenship was extended to the entire free-born male population of the empire as a revenue-raising measure. After that, the Roman Pentagon increasingly looked outside the frontiers for foederati -- barbarian troops living and fighting under their own tribal leaders. Before too long, foederati and their families were being resettled inside the empire in large numbers, and not long after that, accounted for the bulk of "Roman" military forces.
Not surprisingly, these newcomers had their own ideas about how the empire should be run -- and for whose benefit -- and became increasingly less patient with Roman racial prejudices and cultural pretensions. And the rest, as they say, is history.