BAGHDAD - Three months into Baghdad's security crackdown, sectarian violence is back on the rise — this time, in a mixed neighborhood that had previously been calm while Sunni-Shiite strife tore apart other parts of the Iraqi capital. Baiyaa, a middle-class district in western Baghdad, has started to display the fears and flight that have already carved up much of the city, which has become a patchwork of separate Sunni and Shiite enclaves. Snipers have begun appearing in parts of Baiyaa. Many stores have closed, and both Shiite and Sunni families have fled the area in recent weeks. Last Sunday,...