Why Turkey Is Fighting the Kurds Who Are Fighting ISIS

Kurdish fighters have been coordinating with the American military since last October. From cloaked rooms in northern Syria, members of the militia known as the Y.P.G. have relayed intelligence and coordinates for potential airstrike targets to an American operations center hundreds of miles away.The resulting strikes have in turn helped the Kurds seize a broad stretch of territory along the Turkish border from the Islamic State. “The role of the coalition jets has been essential to these victories,” said Idris Nassan, a senior Kurdish official from Kobani.

The United States has sought Turkey’s help in the fight against ISIS since last year. Turkey, which has been a NATO member since 1952 and is considered by American officials to be critical to weakening the Islamic State in Syria, was reluctant to participate. Turkey finally agreed to assist, but with some conditions, including the creation of an ISIS- and Kurdish-free zone in Syria on the Turkish border.In return, the United States will be allowed to launch military operations against the Islamic State from Incirlik Air Base and other bases in Turkey “within a certain framework,” according to President Recep Tayyip Erdogan. American officials have been careful not to offend Turkey and have publicly supported its campaign against the P.K.K..In a statement on Twitter, Brett H. McGurk, President Obama’s envoy to the coalition fighting the Islamic State, said, “We fully respect our ally Turkey’s right to self-defense.”

Analysts say that Turkey’s decision to join the fight against the Islamic State was driven in part by the threat posed by the Y.P.G.’s rapid territorial gains within Syria. Kurds across the region have historically sought an independent state, which the Turkish government strongly opposes. Mr. Erdogan has said that Turkey “will never allow the establishment of a new state on our southern frontier in the north of Syria.”