The Islamic State’s commanders know that these Sunni fighters pose a potent threat. Before moving into northern Iraq in the spring to prepare their breakout offensive in Mosul, they assassinated former Republican Guard officers who had worked with the United States. But that only deepened many Sunnis’ secret hatred of the jihadists.

Gen. John Allen, the retired Marine tapped as Obama’s special envoy in combating the Islamic State, brings several advantages. He coordinated contacts with Sunni tribal leaders in Anbar during the Sunni Awakening, which crushed the insurgency there. He was also one of the most effective U.S. commanders in Afghanistan. In recent weeks, he has been contacted by Iraqi and Syrian Sunni leaders who want U.S. help.

Iraqis and Syrians tell me that U.S. Special Operations forces will be decisive in training the Sunni fighters who can carry the battle into the streets of Mosul, Fallujah and Raqqah. Obama must decide whether this mission is better performed overtly or covertly — but the Americans who will be doing the training will be the same warriors, drawn from such units as the Army’s 5th Special Forces Group.