Yeah. So.. a big chunk of the Marine Corps are hard-right Trump supporters but not nearly all. The Marines are different in that the leadership is steeped in the history and tradition of the Corps from the start of bootcamp. They will know they can be punished for following illegal orders, and they will already know about the last time Marines were called into LA.
In the end, it will come down to SNCOs and NCOs to make the decision because the Marines try to push down "battlefield" decisions to as close to the action as possible. Of any service, I expect your average Marine to be able to make independent decisions in the moment. That may or may not be a good thing.
Marine officer here (Reservist). The officer corps has a LOT of careerist yes-men, if the enforcement of COVID vaccinations is anything to go by. I know a surprising number of Catholic Marines who refused to get COVID vaccinations and had to deal with significant pressures from senior officers over it. I'm not confident that commanders will refuse orders on the basis that they consider them illegal.
EDIT: That said, I concur with my sibling comment that decisions are very much pushed down to the NCO/SNCO on the ground, and I expect that those young Sergeants might actually have more sense and restraint than the O-5s and O-6s (battalion and regiment commanders). Also, military demographics are changing....I bet (but can't confirm with hard data right now) that there is a substantial growth in Hispanics in the combat arms compared to ~30 years ago. I had a female Corporal working for me once who's hometown was Sinaloa (yes, cartel Sinaloa). I think even normally "eager to engage the enemy" Marines will be more restrained about escalating to violence against their own demographics.
I think the more interesting question is what happens when the right-wing paramilitaries (Three Percenters, Oath Keepers, and various assorted local militias) step in, if Trump gives the military orders to stand aside and let them do whatever.
I mean, if that were to happen I'd agree with Trump. We shouldn't be deploying the military against domestic paramilitaries, we should be deploying law enforcement or the National Guard.
