As a government warplane soared over the northern Syrian city of Raqqa recently, a fighter from the Islamic State of Iraq and Syria, the country’s most radical group linked to Al Qaeda, watched from behind an antiaircraft gun mounted on a pickup truck. Fighters and activists from rival insurgent factions urged him to fire. He did not. The others were incredulous, recalled one, who supports the Nusra Front, a rival group that has Al Qaeda’s official stamp of approval as its representative in the fight against President Bashar al-Assad. But the man on the truck replied, “We are here to establish the Islamic state, not to fight Assad.” Such disputes helped set off the infighting that has swept insurgent-held northern Syria for the past week, leaving more than 500 dead, as a broad array of factions have turned against the Islamic State of Iraq and Syria, […]