Soooo why did anyone support this the first time around?
Oh yeah, cause Iraq was gonna nuke us...
Well, there are certain justifications for war and at the time we were justified in invading due to the fact that we did fear an attack from Hussein's Iraq.
Whether or not the war overall is constitutional, or if it's still justified is debatable. However, at the start we had reason to believe that Iraq could harm us. Also, whether or not our intelligence was right in that regard is also debatable.
But there was a legit reason.