Back in 2005, al Qaeda’s current chief Ayman al-Zawahiri wrote to Abu Musab al-Zarqawi, the founder of Islamic State’s predecessor. He chided Mr. Zarqawi for killing ordinary Iraqi Shiites and beheading hostages.
Mr. Zawahiri warned that such wanton killings would erode critical public support, “a decisive factor between victory and defeat.”
Mr. Zarqawi and his successors ignored that advice. Since then, al Qaeda has been dwarfed by Islamic State, which has taken control of vast swaths of territory in Iraq and Syria. Many fighters of al Qaeda’s Syrian franchise, the Nusra Front, have defected to Islamic State.