Why America’s Kurdish allies are under threat in a new Syria

The 13-year civil war between Syria's government and rebel fighters has ended. But the peril is not over for Syria's Kurdish minority.

A number of armed factions are still jostling for control after the collapse of the Assad regime. They include the Kurdish-led Syrian Democratic Forces, which have allied with the United States to combat the extremist Islamic State group, and the Syrian National Army, a militia backed by Turkey, which is hostile to the Kurdish forces.

For more than a decade, the Kurdish-led soldiers have been America's most reliable partner in Syria, liberating cities seized by the extremist group and detaining around 9,000 of its fighters.

But Turkey, which shares a border with Syria, has long considered the Kurdish group to be its enemy. The Turkish government believes the Kurdish fighters in Syria are allied with the separatist Kurdistan...