An al Qaeda spinoff operating near Aleppo, Syria's largest city, last week began a new battle campaign it dubbed "Expunging Filth." The target wasn't their avowed enemy, the Syrian government. Instead, it was their nominal ally, the U.S.-backed Free Syrian Army. A band of fanatical al Qaeda rebels are turning their guns on more secular rebels in an attempt to turn the struggle in Syria into a holy war. Across northern and eastern Syria, units of the jihadist group known as ISIS are seizing territory—on the battlefield and behind the front lines—from Western-backed rebels. Some FSA fighters now consider the...