And once it becomes a caste, it starts drifting apart. Politically, that is fairly obvious if you look at the polls. There's a surprising geographic component to it, too - for one thing, disproportionally many recruits come from specific geographic regions (mostly the South). And there are several municipalities around the country where most people who live there are veterans and their families.
The real problem, though, is that such a military caste starts seeing it as separate and distinct from the rest of the country.
https://inss.ndu.edu/Media/News/Article/1428887/deconstructi...
And that, in turn, allows for sentiments like these to develop:
"I am irritated by the apathy, lack of patriotic fervor, and generally anti-military and anti-American sentiment. I often wonder if my forefathers were as filled with disgust and anger when they thought of the people they were fighting to protect as I am."
If such sentiments become prevalent, how long will the military tolerate civilian control, if it considers the controlling civilian government to be run by people with "anti-American sentiment"?