The President said that Saddam was an imminent threat, and that he needed to be removed before he could use his ability to produce and deliver WMD's to terrorists.
Some have said we must not act until the threat is imminent. Since when have terrorists and tyrants announced their intentions, politely putting us on notice before they strike? If this threat is permitted to fully and suddenly emerge, all actions, all words, and all recriminations would come too late. Trusting in the sanity and restraint of Saddam Hussein is not a strategy, and it is not an option.
The rest of his justification for removing Saddam is in the 2002 SOTU as well.