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.