Like it or not, the legal basis for the war was predicated on far more than WMD. Consequently, it was a terrible diplomatic and later political mistake to focus just on WMD — not just because of what happened afterwards (again, had the occupation gone well, human nature being what it is, the absence of readily deployable WMD stockpiles would not have led to the “Bush lied, thousands died” topos), but rather because the Bush administration itself had invested so heavily in obtaining congressional support for the preemptive attack.
The Congress, with apparent support from the Bush administration, authorized the use of force on the basis of 23 writs, many of them humanitarian and especially appealing to the bipartisan consensus. Yet by de facto fixating on just one cause, the administration subverted the spirit of the authorization, and allowed those who voted for the broad authorization to later claim that they had been duped — even though the vast majority of their own causes for action were completely unaffected by the absence of WMD stockpiles.
Note that a Hillary Clinton or John Kerry did not have to state that, “Because there was no WMD, my reasons for attacking Iraq — Saddam’s sponsoring terrorists, giving bounties to suicide bombers, attacking his neighbors, or conducting genocidal attacks — are now null and void.”