interesting question - there was discussion in early “Q” days that the Marines are under special jurisdiction by POTUS - they provide security and transportation.
I’m pretty sure you are both right. Up until 1999 Marines were stationed on ships (I think I read somewhere that the Marines want to go back to that)
I know the early days they were commissioned to protect the US Mail.
The Marine Corp Manual seems to cut out the middle men. (though I’m sure other laws are in play as “middle men”)
2. Mission and Functions
a. The Marine Corps shall be organized, trained, and equipped to:
(1)-(6)...
(7) Perform such other duties as the President may direct.
I think I read that the other services also had reserve call ups ?
I wonder what they are expecting ? an Operation Wetback type of operation ?