I would shudder to think about our troops firing on unarmed civilians like the situation you laid out. That’s why I’m a big proponent of Posse Comitatus. True, it was not part of our founding documents, but I believe that it is strongly within the spirit of our founder’s (and mine) beliefs.
Our federal troops should only be used to guard the borders at home (IMO). I think that does not violate Posse Comitatus as we are protecting our citizens against outsiders. However, they should not be carrying out any other real world operations within our borders.
Is the “Old Guard” a National Guard Unit? I thought crowd control like that would be a function of a state national guard. It should not IMO be a function of the Army per Posse Comitatus.
Since then and especially after the so called “Kent State Massacre” in 1970, specialty military police units have been formed to handle civilian “situations” such as these. Two such units are attached to TOG in the National Capitol Region.
http://www.army.mil/info/organization/unitsandcommands/commandstructure/theoldguard/