The existence of MP's does not violate posse comitatus. When these guys start directing traffic in your neighborhood and pulling you over for speeding then you can be upset. In 236+ years of the US Army MP's existence no one has hollered foul, why you seem to think that the USMC standing up a few battalions is unusual and threatening is beyond me. Of course, I'm a member of that machine, and not to be trusted, so I don't know why I'm wasting the effort.
^^^This...