Prior to the invasion I formed a strong view which I haven't changed. I thought that Bush wanted to invade Iraq specifically because he knew that Saddam did not have decisive weapony. I thought the fly-bys and inspections were a process of softening up for invasion, and that as soon as they thought the Iraq army was weak enough, the US would invade. I was wrong in one respect - I thought Iraq would resist the invasion harder, rather than doing the Sun-Tzu tactic of letting the enemy in and getting them bogged down. On the matter of evidence, I think Bush saw its veracity as irrelevant, because the conservative world-view is that people support power as authority, and once he had demonstrated power he believed moral legitimacy would follow.
I think Blair on the other hand allowed himself to be persuaded. I think he was quite wrong to do this, he should have been more cynical and suspicious. I've done things like this myself: I know that someone is lying to me, but I persuade myself otherwise, everyone tells me 'don't be so cynical'. I tell myself it will all work out for the best. Blair stupidly thought that Bush would behave honourably, and was let down. I think he believed that the results of the invasion would be so benign that the veracity of evidence would not be a major issue in the general jubilation. I think he was also persuaded because he thought that being rigid about evidence would bring economic and political sanctions down on our country. I think he should have called the American bluff like Wilson did when he kept Britian out of Vietnam.
Incidentally I think the expression 'Weapons of Mass Destruction' was an immediate indicator that things were wrong, for everyone to have spotted from day one. It was a coined term which parcelled together a range of different weapons, the responses to which should have been quite different, for purely rhetorical purposes. A country which had a good case would not have had to do this.