"Ok, got that. Because a tin pot dictator was rattling his sabor, what he says must have been credible and it must have constituted a genuine imminent threat."
Perhaps if Saddam did not possess total control over his Fascist military state, mass executing people wholesale with chemical agents, and then state he intended to direct those same measures against the United States and her allies, I would start to call him "tin pot." Literal or hyperbole, it doesn't matter. Until then, he has and always will be a genuine threat.
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"Ok, so because there are such things as biological weapons, only a small amount of which is needed to be lethal, this invasion was justified."
Absolutely, as was the general war on terror. Failure to act in the past only led to more disastrous outcomes, as shown by the first Trade center bombing. Someone with established history of using WMD says "you're next," and you reply "we'll wait to see if he's bluffing"? I am so very glad you aren't even remotely in a position of authority.
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"the U.S. does not share a border with Iraq, ergo, what might be a threat to Iran from Iraq is not necessarily a threat to the U.S."
Ultimately, your argument formed somehow around "sharing a boarder" is not salient: you wished to show that because of our geographic proximity to the state of Iraq, our forces, personnel, and interests would not be in jeopardy? Please, I expected more from you than that. Iraq used WMD's against a quarter million Kurds, and Iranians during the conflict, and you make the jump to boarder sharing to shoo that little tidbit away? "Yeah right." is not the most accommodating response, but it's the first one that comes to mind.
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Taking seriously the determination of why and when to engage another state is not the issue. People are not in arguments over our involvement with Iraq because one side thinks the other doesn't take that criterion seriously. It's because both sides use the criterion; one side stating that it has indeed been met, while the other does not. You are clearly the latter.