“What we didn’t do was predict the will to fight. That’s always a problem. We didn’t do it in Vietnam. We underestimated the Viet Cong and the North Vietnamese and overestimated the will of the South Vietnamese. In this case, we underestimated ISIL [the Islamic State] and overestimated the fighting capability of the Iraqi army. . . . I didn’t see the collapse of the Iraqi security force in the north coming. I didn’t see that. It boils down to predicting the will to fight, which is an imponderable.”

Intelligence officials haven’t publicly discussed the prospects for success of President Obama’s small-footprint strategy for combating the Islamic State through a coalition of nations, without directly committing U.S. combat troops. But some officials appear wary.

“If I were head analyst, I don’t think I’d make a call yet,” one senior intelligence official said, requesting anonymity. “I haven’t fit together the contributions that each of the coalition members might make.”