I think Matthew Yglesias is right about this
"The point, however, is that 'sensible' opinion on the war has tended to follow extremely superficial trends. When it briefly appeared that the march on Baghdad was getting bogged down by Fedayeen, Rumsfeld was a disaster. When things were calm in April-May 2003, Bush was a genius and the anti-war folks utterly discredited. Then things started getting worse and doubts grew. Then Saddam was found and folks like Howard Dean who said this wouldn't change anything were said to be in a 'spider-hole of denial.' Then things went downhill and everyone said Rumsfeld needed to be fired. Then Iraq had its first election and everyone said the anti-war group was discredited again. Then things went downhill again and now virtually everyone says the conduct of the war has been incompetent."