Greenwald has a post about deploying American troops within our own borders. I rather agree with his conclusion: the immediate move of a single brigade is probably not dangerous in and of itself, but the precedence is bad.
I don't think this is like Julius Caesar's move in taking his own legions from Gaul to Rome, which brought the Republic to an end, because American soldiers would probably not be willing to pressure Senators and Representatives to vote "the right way", unless they were chosen very carefully. I don't think they can be chosen that carefully, yet. But I really don't like the direction we are heading.