Threatening U.S. forces in Manbij—Erdogan has said he would be forced to “bury” them and warned that although “they tell us ‘don’t come to Manbij,’” Turkey “will come to Manbij” — could irrevocably damage the decades-long U.S.-Turkish alliance, which is already under strain from a combination of Erdogan’s deepening authoritarianism, flirtation with Russia and conflicting interests in Syria. A clear U.S. statement that this would be intolerable, backed by an effective deterrent posture, is necessary to prevent outright U.S.-Turkish hostilities and preserve any hope of a functional relationship going forward.

Unfortunately, until now a cacophony of mixed American messages, combined with a longstanding tendency to treat Turkey gingerly, has convinced Erdogan that Washington sees its relationship with Ankara as too important to fail. This only increases his appetite for risk—and thus the potential for conflict. Ankara must be made to understand the dire consequences of attacking Manbij, and Washington must be prepared to follow through.