Ian McEwan interviewed by Der Spiegel:
I never thought that in the run up to the war we were discussing simply the difference between war and peace. We were discussing the difference between war and continued torture and genocide and abuse of human rights by a fascist state. I missed any sense of that complexity in the peace camp. I certainly had the feeling that whatever the strong moral arguments were for deposing Saddam, the Americans would not be good nation-builders. But I had a moral problem with this view among the 2 million protesters that you should leave Saddam in power in a fascist state with 27 million Iraqis under him. The problem is that they felt good about it. I thought they should have opposed the war but also felt bad about it.(Via Mick - h/t RB.)
I think if Bush and Blair could press a button and we could all fast forward backwards, rewind the tape, they'd probably do this differently. But I don't think they fully grasped, and even the anti-war (movement) could have never fully grasped the fantastic viciousness of the insurgency against its own people.