"Given all that, what difference does the existence of WMD make?"
What difference does it make whether the reasons we are given to go to war are true or false, as long as we get to go?
A President lies about his personal life, and he's scum. Another President dissembles about a justification he uses to persuade moderates that a first strike is actually self defense, and he's a hero.
That's the difference.
Regards--
Mary Anne