A Hypothetical Case

The first thing I would do in this situation if I were the President of the U.S would be to try to get more information about the situation. I would turn to my consultants to get to know where there is any other country that has its own interests in the country discussed.

I would request data from the intelligence department to find out whether the revolutionaries get financial, political or some other kind of support from some country, organization or person.

Than I would call the OAS meeting and consult them on a course of action they consider being appropriate in this situation. During it I’ll emphasize the importance of assuring safety of the 200 American citizens that reside on the territory of the country.  I’ll listen to their suggestions, and after that I would call an emergency meeting to discuss the situation with the Senate Foreign Relations Committee.
Depending on the choice I would make after consulting the members of the Committee, I would either try secret negotiations with the rebellions or the political force that stands behind them, or call the emergency meeting with the Senate, where I would place the question for whether the U.S should land its Marines in order to help the previous government.
As the President of the U.S I have the authority to call the emergency meeting of the OAS, the Senate and the Senate Foreign Relations Committee, in order to discuss the situation with them and make a decision. Landing Marines without getting an advice from the Congress might create a conflict with it, and the relations with the OAS would be strained if I recognized the rebelling group as the new legitimate government without calling a meeting with them.
It is also that the force measures taken by the U.S may call negative reaction from international community, and I would pay much attention to this issue, as in the contemporary situation with terrorism the U.S cannot allow itself to loose the potential allies among the other countries.