OK not a main argument against invastion - but a good reply to one argument.
The claim is that Iraq was uniquely evil because of
1) Actually having used WMD
2) Because he used them against his own people.
That we sold them to him at the time he was doing this is a good reply to these - since apparently we did not think these particular things made him uniquely evil at the time. And it is a special point that Cheney was one of the ones who sold stuff to him.
There are other answwers to these points, and other points to answer. But I really think it is one (though not the main valid) point.