The men who snatched Abu Issa from the streets of this southeastern city were Turkish gangsters, but their client was the Islamic State, and they had been promised good money to spirit the Syrian rebel commander across the Turkish border into Syria.

The effort ultimately failed, but the story of the brazen daylight kidnapping and its chaotic conclusion has raised troubling questions about the militant group’s growing reach into Turkey, as well as the capacity of the Turkish authorities to contain it.

Over the past year, this NATO ally has come under intense pressure from the Obama administration to do more to halt the flow of thousands of foreign fighters who have swarmed into the country to join the war in Syria. Turkey says it is trying, and strict new border procedures, along with reinforced patrols, barbed wire and watchtowers, are evident at all the major crossing points into Syria. […]