It was obviously tremendously dumb to leave Iraq, only the most pacifist of antiwar people could disagree. But the people that are to be blamed for it (aside from the people back home that made it allowable politically) are the Shia members of the Iraqi parliament. The US continued to try to negotiate through the early fall of 2011 to stay in Iraq.
I know, I was there, reading the daily reports from the embassy.
And as far as whether or not Iraq itself was a threat...that's beside the point.