Saddam tried to assassinate a former president of the US. He shot at US pilots. He defied the international community. He committed mass murder.
Although we haven't found WMD, we didn't have the burden of proof. Saddam did.
We had the experience of the First Gulf War where the intelligence community had saeriously under-estimated the WMD that Saddam had.
Excellent points by DC, a great job transcribing by Peach!