Seeing as how so many ISIS commanders are Saddam’s former lieutenants and feeds on Sunni resentment, I’d vote for the much praised total withdrawal of American troops by Obama combined with the Shia/Iranian dominated Iraqi government.
Radical Islam has always been around since Muhammad established his warrior religion. So that’s the base cause with the incidents on this list are just markers along the way or perceived markers anyhow.
However, the Sunni resentment is due to the US handing over Iraq to the Shi’ites after our victory. We kicked out the Sunni officials without eliminating them, so they were always going to rise up and take their country back. O’s feckless refusal to engage made it easier for them and made it happen earlier,