ISIS was there before the invasion? Are you mixing up ISIS with Al-Qaeda? Al Baghdadi, the ISIS leader, "graduated" in the |american run prison after the Iraq War. Prior to that, ISIS was a practically non existent group.

https://en.wikipedia.org/wiki/Abu_Bakr_al-Baghdadi

ISIS was a major player against the American invasion? The Muslim fundamentalists hated the secular regime of Saddam.

And adding fuel to the fire was the proliferation of Shia militias after the fall of Saddam. They were the ones who opposed Saddam the most.