On Jan. 20, Turkish forces attacked Afrin, a Kurdish-controlled enclave in northwestern Syria. As airstrikes rain down on Afrin’s mountain towns, the conflict is putting both American and Russian plans for Syria to the test.

Most of Afrin’s original inhabitants are Kurds, though the population, now estimated at 323,000, has swelled with civilians displaced from other parts of Syria and also includes Arab towns seized by Kurdish forces. Apart from government-controlled Aleppo to the southeast, Afrin is entirely surrounded by Turkish territory and Turkey-backed rebels. A previous Turkish intervention in October set up military outposts all along Afrin’s southern border. ...