Alternatively bringing in a flood of foreign fighters who can be abused and extorted into doing dirty deeds by politicians holding an extremely lucrative carrot (US access) and stick (deportation) sounds like a recipe for abuse of the soldiers, and likely anyone they’re deployed against.

Considering the history of the French Foreign Legion, I’m disinclined to believe in the best possible outcome here.

History has taught this lesson.

In Starship Troopers (the book), I'm pretty sure the ptoagonist (was it still Rico?) was Phillipino. Almost wonder if Heinlein might have drawn on this episode as inspiration for his ideas around service for citizenship.

That's an extremely low bar given many of the Americans I've met.

Even in a comparitively similar country like Australia, where I live, Australians have a hard time understanding many basic American values, particularly political ones, in my experience

While I agree that many people would do well to have the kind of civic background knowledge they demand people acquire to gain citizenship, and it's an important part of integrating people into a society, there is also a lot that is acquired by living in a society for a time, experiencing and observing its contradictions and how it manages them personally, understanding how you navigate it and your place between two or more national identities and experiences, etc.

That said, I think there's a place for inviting people in. In America's case though, it seems like there's a huge reserve of people already in American society, both legally and illegally, that can be drawn on first. That to me seems like it should be the priority.

An extremely small outfit with extremely high selection criteria and a gruelling recruitment process, that's specifically structured to handle foreign recruits, and drills them extremely harshly in French language. Which offers a citizenship, that while nothing to sneeze at, is regularly not even taken up by some of its soldiers after they serve.

It's not a viable comparison to signing people up at the border to get US citizenship on any level.

However it could act as inspiration for any efforts to found a similar outfit in the States.

I don't know enough about that to open my mouth much on it, but I'd say civil wars are a special case given they are often fought over specific principles, ergo, have a certain natural selection.

Further, you're talking about a time in America's history where standing armies were dispreferred, which is not what modern American militarism looks like at all.

Finally, as far as "foreign born" people go, as I said, I support the idea of military service being a route for them toward citizenship, I just don't think going down to the border and signing people up there is a good idea.

I think one interesting idea, although you might only be able to do it as a once off without creating too many bad incentives, would be to offer amnesty and a route to citizenship this way.

I don't have a dog in the fight though, I'm not American. The risks of making joining the armed forces a direct route to citizenship in one of the most desirable countries to immigrate to in the world is not negligible though, you need to be careful, lest your armed forces turn into green card mercenaries, with no value base.