Indeed, the rival rebel organizations such as ISIS and the other main jihadist group, the al-Nusra Front, spend more time fighting each other than they do the minority Alawite regime in Damascus led by President Bashar Assad.
These and other events have dramatically changed the shape of the 30-month-old war in recent months, much of it to the regime's advantage.
Some regional analysts and intelligence officials say ISIS, and to a lesser extent al-Nusra, seek to tighten control over territory that stretches from Western Iraq, where ISIS has its ideological center, to northern Syria, which includes vital supply routes along the Iraqi and Turkish borders.
Clashes between rebels of the Free Syrian Army, an alliance of nationalist and secular rebel groups backed by the West and Saudi Arabia, and the jihadists have escalated in recent weeks, particularly since ISIS, with its large force of veterans of Iraq, Afghanistan, Yemen and Libya, appeared in the spring.
This is a situation that Beirut-based analyst Michael Young said the Assad regime "has welcomed and indirectly encouraged.
"That's because, as many observers have pointed out, the Syrian regime has generally avoided attacking the al-Qaida groups, and has even collaborated with them in certain districts.
"This has allowed the jihadists to gain ground and in that way confirm the regime's narrative that it's the last line of defense against extremism," Young observed.
Underlining the multi-tentacled Levantine intrigue that now seems to direct this increasingly complex conflict, a senior officer of an Arab intelligence service postulated the Assad regime hopes the moderates grouped under the FSA "will eventually ask the army for help to fight ISIS."
Meantime, the Islamists have effectively split into two, some aligning with ISIS while more moderate elements have gravitated toward al-Nusra, completely transforming what began in March 2011 as a straightforward rebellion against a brutal dictatorship spawned by the so-called Arab Spring.
These days Saudi Arabia, a longtime U.S. ally, is increasingly pursuing its own strategy in Syria rather than stick to an American game plan.
This has involved boosting the so-called "moderate" Islamists by buying them off with hefty payouts and supplies of weapons to fight the hardcore jihadists who, among other things, want to bring down the House of Saud.
The Sunni monarchy in Riyadh, exasperated by Washington's vacillation about using military force against Damascus, is now more determined than ever to topple the Tehran-backed Assad regime as part of the kingdom's confrontation with Shiite Iran.
This is a regional conflict that has become the predominant focus of the Syrian civil war, with the largely Sunni rebels battling an Iranian-backed autocracy controlled by the minority Alawite sect that's a branch of Shiism.
Washington's current effort to negotiate a rapprochement with Iran's pragmatic new president, Hassan Rouhani, aimed at ending 35 years of hostility, has alarmed the Saudis even more, and they seem determined now to pursue their own plan to eradicate the Assad regime.
Despite the U.S. designation of al-Nusra as a terrorist organization, many now consider the group closer to the mainstream because of its own battles with ISIS.
In recent weeks, the anti-Assad opposition has hardened into two Islamist-led alliances both at odds with the Americans, the ISIS-dominated Azzaz Declaration and the Army of Islam led by al-Nusra.
This has largely marginalized the FSA, once the dominant rebel group under the U.S.-backed Supreme Military Command based in Turkey.
That's left the Americans out on a limb in a highly sectarianized, complex war that threatens to engulf much of the region.
The Russians, Assad's strategic partner, have become a key player. It was their diplomatic ingenuity that effectively blocked U.S. President Barack Obama from launching punitive strikes against Syria following the regime's Aug. 21 nerve gas attacks in the Damascus area that killed 1,400 people, mainly civilians.
But Moscow has its own reason for wanting to prevent a jihadist takeover since that could have serious consequences for Moscow in its own, largely unseen, war against jihadists in Caucasus.
Islamist extremism is more of a threat to Russia than it is to the United States.
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