The United States and its allies are facing another major policy challenge in Iraq. Airstrikes against the Islamic State might unseat the group's fighters in critical areas, but as things stand, troops will be needed to hold and govern liberated territory.

Securing Iraq, therefore, requires a formidable force to be in place, which is why U.S. President Barack Obama's strategy includes rebuilding the Iraqi Army. But doing so will require overcoming three related obstacles: Iraqi leaders' military inexperience; corruption and cronyism; and ambiguity regarding the extent of external support.

When states collapse, their constituent parts sometimes inherit armed forces that are competent enough to maintain minimal levels of governance. This is more often the case when a state breaks up as a result of armed conflict, in which case stability depends on whether the best military leaders are allowed to remain in place.