One reason I think the Marines left was becasue it was over there 90 day mark. After that time Congress would have to authorize them to be there. Plus the UN was finally there inforce. The Marines were there to earlt in the begin because they could move in fast.
In relation to the "Army grunts" saving the Rangers and the Rangers would not like it. I have to disagree. If I was fighting for my life street to street and I knew at the other end of the city were tanks and men ready to save me I would be mad. Yes the Rangers did a good job on there own, but alot of good men died that day. And the only reason was becasue the UN would not help and therefore the 10th Mountain Div. counld not becasue they were under the UN command.