Explaining Dubya's failing Iraq policy in two short paragraphs
While there's many explanations (none exclusive of all others) and every situation is complicated, and Glenn Greenwald makes a good case for one alternative (see here as well), I think this explanation by Josh Marshall for Dubya's Iraq behaviour (and the continuing Republican support for it) is about the most succinct I've seen:
So at the outset it was that Iraq and al Qaeda are connected and either did attack us together (as Dick Cheney frequently suggested) or could in the future (as everyone else did). Then the beginnings of the insurgency were not a problem because we were drawing al Qaeda into Iraq to fight them on our own terms. Then we couldn't leave Iraq because doing so would hand it over to al Qaeda.
As the cycle progressed there was an mounting tendency for the administration to argue that we could not abandon its policies precisely because of the scope of the failure of those policies up to the present point -- a veritable perpetual motion machine of disaster and incompetence. But setting that aside, the enduring pattern has been for the White House to ask us to make our decisions about Iraq not based on what is happening in Iraq but on what happened in New York and Washington on 9/11.