The question is, is a military invasion the best way to prevent such behavior on the part of tyrants particularly given that said dictator, Saddam Hussein, was a beneficiary of American largesse for years? Donald Rumsfeld himself, acting on behalf of President Reagan, handed the guy anthrax in the mid-1980s. We didn't really care what old Saddam did to his own people as long as cheap oil keep comin'.(chinadoll)
I've always found incoherent the argument that "But we gave Saddam [weapons/money/support/etc.] for years!" Our past support of a regime does not excuse whatever atrocities they have committed since then. Or even *while* we supported them. By that logic, Britain couldn't have done anything when Germany invaded France, because they had previously let them get away with invading Poland. If it is wrong to support regimes that commit atrocities, then our past support was at best a compromise of our principles to further some other goal, and at worst an outright mistake. Smacking them down now is just rectifying that past mistake/compromise.(me)
As for the response that it doesn't matter whether we gave Saddam aid or not it was not an endorsement of his policies I've heard that one too. While in China the atrocities of the Pol Pot regime came to light which was a surprise to many Chinese people since Pol Pot was an ally of China's against the Vietnamese. And the answer was the same as provided above: we never endorsed his policies in exchange for material aid which was intended for defensive purposes.(Betzee)
As far as I can tell, the argument here is, essentially: We supported Saddam Hussein in the past, therefore we cannot legitimately attack him now. The problem is, I can't derive the second part of that sentence logically from the first part.
Was our past support of Saddam morally right or wrong? If you say it was right, then you are endorsing mass murder. If it was basically wrong, but excusable due to a greater (perceived) threat (i.e., the lesser of two evils), then conditions have changed, and that excuse no longer holds. If it was wrong, and not excusable, then the sooner we reverse our position, the better.
There are only two ways I can see that being a past "beneficiary of American largesse" can have any possible relevance: It might make him a more dangerous target, since he may possess resources we gave him that could be used against us. Or, if we had a contract, and he had upheld his end, then our invasion would be a breach of that contract (though if that's the case, it must have been a pretty lousy contract to begin with, from our point of view). But I have never heard this presented as either a strategic or a contractual argument against invasion.
Make no mistake: I'm not saying that our past support of Saddam was not an endorsement of his policies. I'm saying that even if our past support of Saddam was an endorsement of his policies, that does not mean we are required to continue endorsing his policies today. I'm saying it has no relevance to our current behavior.
A reductio ad absurdum: During the height of the Cold War, the U.S.S.R. launches a nuclear attack against us. The bombers are in the air, on their way to their targets inside the U.S. Are we precluded from trying to shoot those bombers down because we were allied with Russia during WWII? I would say we are not.
Saddam's regime was at some point (we can argue over precisely where) along a continuum from smiling happy friendship, to launching a nuclear attack. There must be somewhere along that continuum where we are allowed to reverse our earlier position of supporting the regime. We can argue over precisely where we draw that line, and we can argue over which side of the line Saddam was on. But to deny that such a line exists strikes me as simply delusional.
So I suspect that bringing up past support of Saddam is nothing more than America-bashing: Oh, look, we're hypocrites. That's fine, as far as it goes (I like Green Day as much as the next punk), but it's not a rational argument. If anything, it's a kind of ad hominem. It's an attempt to delegitimize America's current actions by bringing up past behavior, in which America either compromised its principles, or simply exhibited bad judgment.
There were, and are, several legitimate arguments against invading Iraq, but this is not one of them.