By your logic we should support Assad. Actually, that would be a far easier way to end the civil war than by resorting to any other outcome.
Still, though, regardless of what you think of the US invasion, at the time when Obama foolishly withdrew US troops and turned Iraq over to the completely incompetent and utterly sectarian Maliki, there was no ISIS in Iraq. While the invasion destabilized Iraq, the troop withdrawal destabilized the fragile peace we had managed to piece together.
It was in December of 2008, when he dodged a flying shoe. Obama actually extended the timeline set by Bush and the Iraqis. And for the people who think it would have so simple for Obama to get an acceptable SOFA from the Iraqis I have to ask this: Why the hell didn't Bush just do it, then?
As to there being no ISIS in Iraq at that time, I think you well know they were AQI then.