Both ISIS and the Kurds also tend to rate the Peshmerga’s spiritual force as greater than the Peshmerga’s physical force, while they rate the Iraqi Army’s spiritual force to be significantly less than its physical force (except for Kurdish members of the Iraqi Army, who tend to rate Kurdish units of the Iraqi Army the way ISOS and other Kurds rate the Peshmerga). ISIS and Kurdish ratings of the PKK and YPG are closest to ratings of ISIS; these three forces are viewed as the most spiritually driven of all.

In other tests, we find that the more the people believe in the enemy’s spiritual force the less they are inclined to make costly material sacrifices against the enemy. The Islamic State’s perceived spiritual force seems to intimidate, even paralyze some of its adversaries.

For Kurds on the front line, their tendency to perceive their own spiritual force to be on a par with the Islamic State translates into expressed as well as actual unwillingness to make costly sacrifices in the fight against ISIS, unless they perceive ISIS to threaten directly their sacred value of “Kurdeity.” Perceptions of lack of spiritual and physical prowess with regard to Sunni Arabs fighting ISIS (whether as Iraqi Army or as militia) is associated with perceived lack of willingness to fight to the death, and especially if the fight is seen as an attempt to preserve Iraq rather than recover traditional lands. As one Sunni Arab colonel told us as fighting raged: “I fight zero percent for Iraq and 100 percent for our Sunni land.”

These findings give sense to the actual course of fighting and the final outcome at Kudilah—an outcome that is arguably as relevant to the effort to retake Mosul and degrade the Islamic State as coalition planners thought it might be, but in a very different way than imagined.