BAGHDAD - The Sunni and Shiite residents of western Baghdad's Hurriyeh neighborhood have lived in harmony for years. Their families intermarry. They attend each other's weddings and funerals and pray in each other's mosques. It is a calm area too, with not a single attack reported against the coalition forces since April. That coexistence, however, came to an abrupt end early Tuesday morning. An explosion beside a Sunni mosque killed three people and ripped the fabric of communal unity that bound Shiites and Sunnis, exposing the deep-rooted sectarian divisions within Iraqi society. The Sunnis blame the explosion on militant Shiites...