I'm getting rather tired of this old wheeze. Can this author show me how that, because we desire for a number of reasons to overthrow one dictator, this somehow commits us to overthrowing every dictator in the world? In point of fact it does nothing of the sort. Changing a tire on your car does not obligate you morally to changing all the tires that are flat in the world.
This applies because one of the arguments put forth (amid the many) for the invasion of Iraq is that Sadaam is a cruel and murderous dictator. Therefore, if this is reason for war, then it stands to reason that all murderous, cruel dictators are subject to forceful removal by the US military. So to go back to your analogy, the Bush administration is saying that your tire must be changed because it is black. This begs the question - why not change all tires? In other words, Vin's argument has validity because the Bush administration used the excuse in this way. If they hadn't, that would be a different story.