So, 2011 the end of 2011, when the US military left the country, AQI, which was the predecessor to ISIS, was at its weakest point, it was at its nadir. As soon as we left, they started to rebound. They started to rebound because the military pressure came off of them, and they started to rebound because Prime Minister Maliki, without the US there to babysit him, started making a whole rash of bad political decisions which alienated Sunnis
The bottom line is AQI would never have rebounded had we stayed there in some number.
Funny how when the Iraqi army was running away in 2012- 2014 the white hut blamed Maliki. So they installed a new President of Iraq ( like the US has such a great track record in installing potentates to replace other potentates) and now there is ..... Silence.