On Oct. 9, Turkish President Recep Tayyip Erdogan finally confirmed speculation that had been swirling for months: Turkey was beginning major combat operations in northern Syria with the goal of creating a safe zone to eradicate the presence of the Kurdish People's Protection Units (YPG) and the Islamic State. The issue is complicated, however, by Washington's tactical partnership with the YPG (which rebranded itself as the Syrian Democratic Forces, or SDF) as a joint means of terminating the Islamic State. Wishing to move against the Islamic State, the Obama administration hit upon what it deemed an effective and acceptable approach, committing a limited number of U.S. special forces and airpower and relying on the YPG to conduct the bulk of combat operations against the group. But even if Turkey's contempt for the YPG was always evident, why has it chosen to act unilaterally against the Syrian Kurds now? Ultimately, the...