Explain the basis of power of the totalitarian dictator and from whence his "right" to hold power derives.
No, don't bother.
Since the totalitarian dictator takes his throne by force at gunpoint, anyone with the ability to use force to displace him has an equal right to do so if they wish. Ergo, if the United States can overthrow a dictator by force of arms, we have a right to do so greater than the dictator's right to retain that throne, since the dictator himself established the foundation of his authority upon the dictum that Might Makes Right.
Welcome to the real world. We have the "right" to overthrow any totalitarian dictator we wish to bump off, by their own example. That we choose to refrain from doing so is merely indicative of our own right to choose to act in our own interests.
You've never understood the proper application of power to politics and diplomacy, have you?
So, to provide the answer to the question you dodged, is that the dictator himself, in this case Saddam Hussein, provided all the justification we needed to overthrow him at the time of our choosing.
Just in case you didn't notice, the United States was founded upon a profound disprespect for kings and tyrants.
Since I'm not a socialist, I still don't respect kings or tyrants.