“What would stop them from doing that to a bunch of 1st Mountain Marines on the sly, and then sending them to these cities as the “Federal Marshals”?
The law.”
Don’t mean to be a pain, but which law? Posse Comitatus or something else? My understanding is that PC does not apply to the Marines. If there is another law, why wouldn’t it apply to National Guard troops also?
The USN/MarCorps aren't covered by PC. There are other statutory and regulations that forbid their participation in these types of festivities, because the AD are Title 10 forces.
NG if federalized are Title 10 forces as well, and then PC would apply to them. Currently the deployments of NG you see are under Title 32 and while paid for by the Feds, are under state command and control.
My understanding is that PC does not apply to the Marines.
You are correct, but it doesn't matter because of the above.