You are ignoring important points.
First and foremost, al Qaeda in Iraq did not exist until we invaded. It sprang up soon afterwards, though. This is important because it later became ISIL then ISIS. It wasn't a problem until it invaded Iraq from Syria. The opposing Iraqi army fled, leaving a bunch of vehicles, arms and supplies behind. This was crucial because any other militia that had gotten their hands on that stuff a) wouldn't know how to operate it and b) wouldn't know how to use it effectively. Because ISIS had people with all levels of military training they could do both. This is crucial because that and their sudden explosion in their ability to recruit and attract money that made them dangerous.
That is what made them important. The military we had spent so much money to train and equip, fled before first contact with the enemy, leaving behind all of those expensive supplies we had given them placing them in the hands of people who knew how to use them because we had kicked them out of the army and gave them nowhere else to go.
Now granted, we don't know if they would have stayed with the army in an occupied Iraq. Maybe they would have left anyway. Maybe they would have tried to moderate the Shiites, successfully or not. What we do know is that we kicked them out and did nothing to make sure they didn't take their knowledge somewhere they could do damage. I suppose Bush just figured they would vanish, but they didn't.
You are ignoring that ISIS didn't exist until we invaded. And existed only because we had invaded, like all of those militias that popped up after went in. If Saddam had still been in power, he might have formed something ISIS-like to cause trouble in Syria and, by extension Iran. But they wouldn't have tried to invade Iraq, their benefactor and even if they did, it is not likely that the Iraqi army would have fled leaving all of those arms in their hands. So your speculation is totally without any basis at all.
It will be amusing to see how you try to spin this. |