Air strikes against ISIS in Syria would be of questionable utility without strong moderate allies inside the country. And they could be viewed as a move to aid Assad.

In Iraq, meanwhile, Obama has been reluctant to provide direct military assistance to the government of Nouri al-Maliki, the Shiite prime minister, whose authoritarian and sectarian-tinged policies helped to spark the Sunni insurgency that let ISIS surge in Iraq in the first place. Maliki’s forces have been reinforced by Iran-controlled Shiite militias, and they have also been condemned for deploying barrel bombs over civilian areas, making it difficult for the U.S. to support them. As ISIS and allied Sunni militants marched toward Baghdad in June, the U.S. responded by sending military advisors and pushing for Maliki to step down.

The Kurds, however, make for more palatable allies. They boast a militia — called the peshmerga — that are secular-minded and also well-organized. The Kurdish region is also home to major U.S. oil companies and has long had close ties with Washington. Many of the Iraqis who have fled other parts of the country amid recent ISIS advances have taken shelter in Irbil, which had been relatively stable and secure since the U.S. overthrew the regime of Saddam Hussein.