MANBIJ, Syria (AP) — A rowdy mob of over two dozen men on motorcycles streamed into the small square in the center of Manbij. They cheered and waved jerseys of their local soccer team, which had just beat the town's top club. The upset victory was sweet, and their wild chants reverberated, "My people! My people!"

It was a moment of normalcy distracting from the tensions engulfing the northwestern Syrian town, which is at the heart of a tussle among Syria's Kurds, the United States and Turkey.

Turkey has threatened to march on Manbij and wrest it from Kurdish hands after its forces won a resoun...