I believe the justification was their non compliance with UN inspections. If you don't let us in to look for WMD, we will break down your door and take a look. If we didn't find any, good, but you should have just let the inspections happen and you could have avoided the whole thing.
Sad to say, but to me when I hear WMD, I think of Colin Powel giving his presentation to the UN showing all of the pics and diagrams and vials. THOSE are the WMD's that were never found, and the reason that so few people trust W.