Few wars have seen such a tangle of combatants as Syria’s, from obscure and morphing rebel groups to Russians, Turks, Kurdish and Iraqi militias. From the chaos, one clear winner is emerging.

Returning to his ancestral Syrian town of Qusayr after years away, a man named Mohammed discovered a new militia patrolling the neighborhood. Patches on the men’s camouflage uniforms called them the Islamic Resistance of Syria. Their identity became clearer when he found a notice on his house claiming it for Hezbollah, the Lebanese militant...