Responding to a point in the comments section:
Andrea, it is not only possible to determine legitimacy (and to leave aside the quotation marks), but the question of legitimacy is vital simply because of the problems involved in one nation interfering in another's affairs and the legitimate use of power that is associated with that. Part of the reason the United States doesn't mess with the affairs of other nations is because of this sort of understanding, which dates back to before the incorporation of the United States themselves. (It was a reaction to the religious conflicts that preceded the current state system that culminated in the Treaty of Westphalia, the event I mentioned in my post).
As for public opinion? Foreign affairs and international relations are not usually that affected by public opinion in the United States simply because the public is fairly disconnected with both. Indeed, the foreign policy of different administrations tends to blend together, because the partisan concerns of each president tend to melt away in the face of geopolitical realities. One of the biggest differences about the Bush II administration from previous ones is how unaffected they are to this sort of influence; the conflict between the "doves" and "hawks" in the administration is fundamentally between those who want the United States to adhere to its normal foreign policy and those who want a much more activist (leftists usually call it imperialist) foreign policy that fundamentally reshapes the geopolitical shape of the world in order to suit the United States' interests.