Last June I visited "checkpoint zero," marking the frontline just outside the city of Manbij, Syria, where an Islamic States-claimed suicide attack killed four Americans on a routine patrol last Wednesday.
"Checkpoint zero" is manned by the U.S.-backed Syrian Democratic Forces, the alliance of Kurdish, Sunni Arab, and Syriac Christian militias and other forces created in 2015 to defeat the Islamic State.
However, there isn't the Islamic State on the other side of the frontline that "checkpoint zero" defends, but fellow Syrians; rebels backed by the Turkish military. The Euphrates Shield, as Turkey named them.
They are there to prevent Kurdish-led Syrian Democratic Forces from expanding further west of the Euphrates and toward the border with Turkey--a scenario the Turkish government considers as a threat because of what it claims is an existential link between the Kurdish fighters in Syria and the PKK in Turkey, who has been engaged in a bloody armed struggle with the Turkish state.
The Kurds and their Syrian Democratic Forces allies said they have every reason to fear that Turkey and allied Syrian rebel forces will act in Manbij the same way they did in Afrin, a Syrian city further north, where Turkish-backed rebels reportedly committed exactions against the local population.