First and foremost, the British. The problems facing Iraq and Syria precede the Obama administration's withdrawal from and the Bush administration's entrance into the country. In fact, neither president was yet born when the British almost singlehandedly created the borders of the modern Middle East.
This is not to say that neither president's policies enabled ISIS, but that Iraq and Syria's problems did not begin 11 years ago.
Desperate for WWI allies and concerned with the preservation of its empire the British made a series of contradictory promises, nearly 100 years ago, that can explain many of the problems facing the Middle East.
First, in exchange for staging a revolt against the Ottomans, the British promised the Hashemites an independent Arab state. To give the Jews an escape from persecution in exchange for their support, they promised a Jewish homeland. Meanwhile, a secret deal with the French divided parts of those same territories into British and French spheres of influence.
Kurds, Sunnis and Shi'ites were combined into the state of Iraq while Sunnis were divided along the Syria-Iraqi border, in both cases, with no regard to the wishes of their populations.
Certainly, ISIS' chilling actions are rooted in a twisted, fundamentalist view of the world. However, their fixation with eliminating the Iraq-Syria border to establish an "Islamic State," is, by historical standards, not radical (it is their violent, reactionary ideology that is).
Nor is the possibility of dividing Iraq into separate, fully autonomous Shi'ite, Sunni and Kurdish regions, or perhaps even separate states.
This is not to say that the root of the problem is religious in nature, because it is not. Rather, it is political in that various groups are vying for power, influence and a decent livelihood.
Such a division could help to alleviate sectarian tensions and competition for power. It could diminish ISIS' ability to exploit Sunni grievances against a Shi'ite central government. It could cultivate more genuine nationalisms that reflect common identities and better resist violent ideologies.
Dividing Iraq would certainly present a host of new problems but it should not be dismissed outright. The "Balkanization" of the Middle East could be the start of greater stability -- not unlike the aftermath of the breakup of Yugoslavia after its own brutal sectarian wars.
The problem with ISIS is partially rooted in historical missteps and the international community should recognize and account for them when considering appropriate policy.