I think the troops were there to "conquer" the hostile force. It was a symbolic act. Most likely the symbolic act was required by some law or rule to inform the world that the US Military is now in control.

I remember the first day that everything was closed off a reporter complained that he needed a passport to LEAVE the area. I never heard a thing about that afterwards. That spoke volumes.

Too me that said that the fenced in area in Washington DC was not part of the US at that point in time.