(CNN) After a prolonged spell of silence, al Qaeda leader Ayman al-Zawahiri has come alive with a series of audio messages. And they're raising a few eyebrows because -- in two separate messages -- al-Zawahiri has extended an olive branch to ISIS, even while describing ISIS leader Abu Bakr al-Baghdadi's "caliphate" as illegitimate.
For nearly two years, al Qaeda and ISIS have fought an unusually public battle for supremacy in the global jihadist movement. Al Qaeda disowned ISIS early in 2014 because al-Baghdadi ignored its directive to stay out of Syria. And its affiliate in Syria, Jabhat al Nusra, is viscerally hostile to ISIS.
Now al-Zawahiri has urged all jihadists in Iraq and Syria to cooperate in the face of a common enemy. And if his words are heeded -- though it's a big "if" -- they could herald a real change on the Syrian battlefield, where the Bashar al-Assad regime has benefited from perennial infighting among rebel groups.
It's not clear when al-Zawahiri's messages were recorded, but the first, posted on September 9, included references to Taliban leader Mullah Omar without acknowledging his death -- and appears to have been produced in March or April.
In it, al-Zawahiri speaks at length about how al-Baghdadi has split the Muslim community, the "sedition that al-Baghdadi and those with him seek to raise among the ranks of the mujahideen ... assigning themselves as guardians of the Muslims without consultation" in declaring the caliphate.
In fact, much of the 45-minute recording is spent berating al-Baghdadi. "We do not acknowledge this caliphate," Zawahiri says, according to a translation by the SITE Intelligence Group. "Instead, it is an emirate of taking over without consultation, and Muslims are not obligated to pledge allegiance to it."
A truce or, worse, cooperation between al Qaeda and ISIS would be bad news for the Assad regime, the United States and many rebel groups in Syria. In a briefing issued Sunday, the Institute for the Study of War says that "tactical or operational cooperation between ISIS and al-Qaeda is a dangerous scenario that may lead to a spike in global terrorist attacks against Western targets and could significantly undermine coalition attempts to regain terrain from ISIS in northern Syria."
In his speech released Sunday, al-Zawahiri called on Muslims in the West to launch attacks on "the homes and cities of the crusader West, and specifically America."
Any response from either al Nusra or ISIS to al-Zawahiri's lectures should provide an indication of whether -- yet again -- his words will fall on stony ground, or whether there is room for coexistence and cooperation.