Welcome to A2K, blubomber and JJ.
Bush is solely responsible because Presidents, more so than anyone on the planet, bear the enormous burden of being certain that, before sending the soldiers of the world's most powerful army off to die, that the reasons for invading are unmistakably clear and valid.
Not "intrepreted", "not "construed", not "the best we knew at the time", not "well, it was still the right thing to do despite the fact that those other reasons we said turned out to be invalid".
The fact that Bush lost no sleep over the decision (as it was reported at the time); that he continues to maintain that his cause was just, that the ends justify the means, so to speak; that he tells other world leaders that "God told him to do it", is indicative of his failure to accept responsibility for the consequences of his actions.
So we, the American electorate, must now hold him responsible.
One need not be a politician nor a statesman to arrive at that conclusion.