The Shrub was an idiot who didn't want to listen to his generals, or it was his intention all along, I don't know which and it doesn't matter, really. Regardless of the public rhetoric DoD knew what was coming. Those guys aren't stupid even if they're sometimes portrayed that way for political reasons.
(Sorry, EagleEye, I have to disagree with you on this one. I think DoD pretty much knew what was coming by invading Iraq and explained it to Shrub, who ignored them. They may not have counted on the high number of "terrorists" that joined in, which probably did delay things a year or two beyond expectations, but they knew it wasn't going to be a 24-month operation.)