The proximate cause of Iraq's unraveling was the increasing authoritarian, sectarian, and corrupt conduct of the Iraqi government and its leader after the departure of the last U.S. combat forces in 2011.
Being in combat is not unending high-five moments.
We can put a stake through the heart of Islamic State as an army. We can put a stake through the heart of its leaders. You can take away its territory. But you can't put a stake through the heart of the ideas, of the ideology, that sadly, tragically, still has some attraction for some small numbers in the Islamic faith.
In counterinsurgency operations, the human terrain is the decisive terrain.
Iraq does have many, many different tensions.
The central problem in Syria is that Sunni Arabs will not be willing partners against the Islamic State unless we commit to protect them and the broader Syrian population against all enemies, not just ISIS.