But in recent weeks there have been signs of a mood shift between the rival jihadis amid efforts to broker a rapprochement championed by some al Qaeda veterans linked to AQAP. And there has been evidence of increasing operational collaboration in Syria with recruiters and people smugglers working for both groups at the same time. Al Nusra and ISIS-aligned fighters have cooperated in the Qualamoun, the mountainous region along the Lebanese-Syrian border, where both groups have been battling Hezbollah, the radical Lebanese Shia movement supporting the government of President Bashar al-Assad.

Last November, the Khorasan network of al Qaeda veterans in northern Syria—they were targeted in two waves of U.S. airstrikes last autumn—brokered a meeting between al Nusra leaders and ISIS, according to senior members of other rebel factions. Al Nusra’s overall commander, Abu Mohammad al-Julani, attended the meeting.

According to the rebel sources, Khorasan was an outlier on the rapprochement front, seeing a role for itself in securing an end to the internal conflict between the archrivals. The Khorasan veterans have close ties to AQAP’s overall leader, Nasser al-Wuhayshi, an authoritative figure across al Qaeda, who was appointed by Zawahiri in 2013 as his deputy and is considered his likely successor.