BEIRUT — For American troops posted in the dusty flatlands of northern Syria, the Palace of the Princes restaurant in Manbij offered a pleasant place to stop for grilled chicken, French fries or its locally renowned shawarma sandwich.

The Americans liked the food so much that they dropped in frequently, often many times a week, residents said. Visiting officials were welcomed to red booths and water pipes; two American senators dined there in July.

“They stop here for chicken and shawarma whenever they have a patrol in the city,” said Jassim al-Khalaf, 37, who sells vegetables nearby. “People here are used to it, so it’s not a new thing to see them.”

The jihadists of the Islamic State noticed, too, dispatching a suicide bomber who blew himself up at the restaurant on Wednesday, killing at least 15 people, including four Americans: two service members, a Defense Department civilian and a military contractor.