Peter — I’m not sure I know either. But I would make a couple points. First, if you’ve got an administration divided between Republican multilateralists, realists and neocons the WMD issue really might be the only consensus position. Bush might also have agreed to play down the other reasons in order to keep Blair and the rest of the coalition in line as well as possibly seduce other nations. Surely the French have no interest in liberating anybody. And the United Nations can’t buy into a plan to redraw the face of the Middle East. Maybe Steve Hayes forthcoming book will have the answer?
But it should be pointed out that the more Bush digs in on WMD the more preposterous the “Bush lied” mantra becomes. If Bush knew that there were no WMDs, the metaphysical stupidity of focusing on it as the prime motive becomes ever more stark, especially if you go by the Kennedy hypothesis which says that Bush did this all for political gain. If he knew they didn’t exist, he might as well have said “We are going into Iraq to stop the slaughter of the unicorns. When we liberate Iraq we will find herds upon herds of unicorns, some will be dead but many will still be alive — if we act quickly!”
So maybe, Bush figured that the WMD issue was the most concrete thing to base the war on. After all, liberation, spreading democracy, fighting al qaeda, lessening dependence on Saudi Arabia, including pulling our troops out of there, were all valid reasons for the war, but Bush could not guarantee their realization upon completion of the war (or by election time). If he was sure about the WMD, he could say “See, we achieved our objectives.” The rest would be gravy.