Matters not. The Constitution clearly states that only Congress may declare war. The president can request it, but Congress must do the actual declaration. Just because every president since Roosevelt has failed to do this, that doesn't make it right for Jr to do it too.
As I stated above in
Message 112. Congress approved the "Authorization for Use of Military Force Against Iraq" Resolution of 2002 (Public Law 107 - 243) which authorized the use of military force against Iraq.
Whether it is justifiable is up for interpretation but the POTUS did get congressional approval for the Iraqi War, whether you like it or not. Personally, and off the record, I do not agree with the justifications for the Iraqi War and thought it to be a piss poor strategic and tactical move after 9/11.
However, you would have a very hard time using his announcement of the commencement of the Iraq War on TV in March 2003 after congressional approval as grounds for impeachment.
Also, the POTUS does have the ability to quickly mobalize the military in limited military operations (less than 60 days occupation) as needed and back fill congress as authorized by the War Powers Resolution of 1973. However, yes, only Congress can authorize a full out war.
Past POTUS have walked the fine line of either intentionally or unintentionally misinterepreting legislation by Congress which approved limited military operations but was short of true "Declerations of War"; to illegitimately utilize the US military to fight what by all common sense should really be considered WARS requiring "Declarations of War" i.e. Korean, Vietnam, Iraq etc. This is really a judicial grey area between the branches of US Government which has yet to be fully clarified.
One of the saddest lessons of history is this: If we've been bamboozled long enough, we tend to reject any evidence of the bamboozle. We're no longer interested in finding out the truth. The bamboozle has captured us. It is simply too painful to acknowledge -- even to ourselves -- that we've been so credulous. - Carl Sagan, The Fine Art of Baloney Detection
"You can't convince a believer of anything; for their belief is not based on evidence, it's based on a deep seated need to believe." - Carl Sagan
"It is far better to grasp the Universe as it really is than to persist in delusion, however satisfying and reassuring." - Carl Sagan, The Demon-Haunted World