(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 call for cooperation
But there’s another theme: restoring unity. Al-Zawahiri and other al Qaeda leaders have previously called for ISIS to return to the fold and accept the leadership of al Qaeda. In his new message, al-Zawahiri says al Qaeda has shown great restraint in an effort to “stop the fighting between the mujahideen” in Syria and to “give room for the people of goodness to reconcile.”

But he goes further, saying that despite the grave mistakes of al-Baghdadi and his supporters, “were I in Iraq or in Sham (Syria), I would cooperate with them in fighting the crusaders, the secularists, the Nusayris (Alawites) and the Safavids (Shia) … because the matter is bigger than me and their claim of establishing a caliphate.”

Al-Zawahiri returns to the theme in the second lecture in a series billed as “The Islamic Spring,” released Sunday. He says “unequivocally that if there is fighting between the crusaders, the Safavids, and the secularists, and any group from the Muslims and the mujahideen, including the group of Abu Bakr al-Baghdadi and those with him, then our only choice is to stand with the Muslim mujahideen, even if they are unjust to us.”

“We call for cooperation with Abu Bakr al-Baghdadi and his brothers,” al-Zawahiri says, “to push back the attack of the enemies of Islam.”

Al-Zawahiri then enters a caveat that somewhat muddies the waters. According to the SITE translation, he adds: “We are not with them if they evade being ruled by the Sharia, or slander the Muslims or brand them as infidels, or break their promises, or seek to split their ranks” — accusations frequently leveled by al Qaeda at the ISIS leadership.

Possible motives
It’s possible that al-Zawahiri is trying to peel off rank-and-file ISIS members, calling them to return to the “straight and narrow” as represented by al Qaeda.

“We consider that most of the corruption in the movement is within a small ruling minority that mixed good work with bad,” al-Zawahiri says.

Perhaps he is also laying the groundwork in case al-Baghdadi is killed and a more conciliatory figure takes over as ISIS leader.

But the impact of his remarks, and the uncertainty about when they were recorded, make gauging their impact difficult. There is no love lost between ISIS and al Nusra, which rejected al-Baghdadi’s attempt to take control of it in 2013.

A leading al Qaeda figure in Syria, Sanafi al-Nasr, who is close to al Nusra’s leadership, has been excoriating about ISIS’ behavior. Al-Nasr, a Saudi with a long track record in Afghanistan and Pakistan, signed a statement in July by foreign fighters belonging to several groups in Syria. It criticized al-Baghdadi’s forces as a “renegade group” which had “increased in their crimes and they bombed the mosques of the Muslims and killed those who were bowing and prostrating in prayer.”

The statement also accused ISIS of stabbing other jihadist groups in the back by launching an offensive against them just as they were beginning a campaign against regime forces.

Al Nusra also cooperates across Syria in various “operations rooms” with other groups. It formed a joint front with several other factions, called Jaish al-Fateh, in the northern province of Idlib. One of those groups is Ahrar al-Sham, whose leadership has been devastated by ISIS suicide attacks.

As Charles Lister points out in a current article for the Combating Terrorism Center’s Sentinel, al Nusra has worked hard to integrate itself with other like-minded groups in Syria, though there are frequently tensions between them. It’s unclear what advantage al Nusra would gain from a broader rapprochement with ISIS, even if locally fighters of the two groups have been known to cooperate.

‘A dangerous scenario’
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.

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