BEIRUT — In the cramped, working-class neighborhood of Tariq al-Jdeideh, Beirut’s last pocket of Sunni control, the walls tell the story of a community in the throes of a potentially profound transformation.

Faded portraits of the mainstream Sunni leader Saad Hariri, left over from the last crisis that engulfed Lebanon, are gradually being eclipsed by a newer symbol, the black flag inscribed with the Islamic creed that is often linked to Al Qaeda.