I've tended to take the darker view: that the war's planners knew it was going to be a tar pit, but lied us into it with promises of "quick in, quick out" and such (as well as the active WMD program hoax). Possible motives range from religious nuttery to just wanting to line the pockets of contractors.

I've suspected religious nuttery for a while because there was just so much of it in the Bush administration. I think a lot of more secular minded folks and religious folks who aren't fundamentalist Bible-belter types just don't get how seriously those people believe in that stuff. The end times are just around the corner. Jesus is coming back. Oh, and the Earth is 6000 years old. They really absolutely believe every bit of it. It's not just a rationalization or a smokescreen, at least not from the point of view of the true believers. (Others may certainly use it that way.)

It was likely a mixture of the two: contractors manipulating the belief systems of the fundamentalists in the Bush administration to profit from a long drawn-out war.