But the recent air strike on a group referred to as "Khorasan" in an area west of Aleppo is a sharp reminder that ISIS is not the only — or even the most dangerous — terrorist threat in the region.
According to analysts, Khorasan is believed to be an elite, militant al-Qaeda cell made up of seasoned fighters from Afghanistan, Pakistan and Yemen who were dispatched by the core leadership to form a base of operations in Syria.
Experts say the name , used by the U.S., might also be used by jihadists outside the region, even though the fighters do not call themselves Khorasan.
Talking to reporters last week, Lt.-Gen. William Mayville, director of operations for the U.S. Joint Chiefs of Staff, said the cell was not embedded to fight Syrian President Basharal-Assad's regime or work towards an Islamic state, but is "establishing roots in Syria in order to advance attacks against the West and the homeland."
That highlights a key difference between al-Qaeda and ISIS, according to Scott Stewart of global security and intelligence firm Stratfor.
It also hints at the underlying dynamic of the two terrorist organizations, and why al-Qaeda continues to have a presence in the region.
Al-Qaeda still poses more of a threat to the West than ISIS because of its focus on transnational terror attacks, Stewart said, because they are strong in "terrorism tradecraft."
He said al-Qaeda elite warriors work clandestinely and could move covertly into another country and make bombs or orchestrate other types of attacks. ISIS, on the other hand, is more adept in warfare and insurgency.
Establishing the caliphate
"[ISIS's] concept right now is to establish a physical Islamic state and that was really not what al-Qaeda was meant to be. They were always supposed to be, or intended to be, a vanguard organization, kind of leading to this other thing. They were never supposed to be the Islamic state themselves,” said Stewart, Stratfor's vice president of tactical analysis and a former U.S. Army military intelligence officer.
Or at least not right away.
In July 2005, current al-Qaeda leader Aymanal-Zawahiri sent a letter to the head of al-Qaeda in Iraq, AbuMusabal-Zarqawi, whose group was the predecessor to ISIS. The letter detailed al-Qaeda's intended goal: the establishment of a caliphate.
However, al-Zawahiri's letter urged "incremental goals" to attain it. He proposed starting by terrorizing the U.S. and the west, and then ensuring popular support among local populations, creating smaller, individual Islamic territories before declaring a larger caliphate. He also warned about overwhelming supporters with too much brutality.
Al-Zarqawi and his successor (and current leader of ISIS), Abu Bakr al-Baghdadi, embraced that vision and ran with it. Al-Qaeda in Iraq eventually became ISIS and sought to expand its operations into Syria. In a public tussle, al-Zawahiri ordered al-Baghdadi to stay confined to Iraq, al-Baghdadi basically said no and so al-Zawahiri broke off all connections between ISIS and al-Qaeda.
But ISIS blazed its way through Iraq and northeastern Syria and declared an Islamic caliphate, thereby attracting a wave of foreign fighters, who see it as an "up-and-coming movement," Stewart said.
"The problem is [al-Zawahiri] really lost control of these guys," Stewart said.
A crisis of leadership
ISIS's sudden stature within the jihadi movement has given al-Qaeda cause for concern, according to Nicholas Heras, a Middle East researcher for the Centre for a New American Security. He said that might be why the Khorasan group was deployed to try to hit Western targets.
"There's that competition in the narrative that's at play and that's what makes [al-Qaeda and ISIS] both dangerous."
DaveedGartenstein-Ross, a counterterrorism scholar at the Washington, D.C.-based Foundation for Defence of Democracies, said while divisions are "extraordinarily large" at the leadership level of ISIS and al-Qaeda, it's less discerning at the foot soldier level.
"A lot of them are jihadists or people who are craving action. They don’t necessarily care as much about which organization [they join] and you have had defections, actually, both ways in recent months," he said.
"During ISIS’s surge, you had defections from al-Qaeda’s affiliate in Syria, [al-Nusra], over to ISIS when ISIS was the stronger horse.”
But in recent weeks, Gartenstein-Ross said, there have been defections back from ISIS over to al-Nusra in Homs and other parts of Syria.
He argued that if the U.S. and its allies continue to launch air strikes on ISIS targets and weaken it, such defections back to al-Qaeda will continue.
"The declaration of the caliphate was a huge mistake," he said, because now ISIS has to maintain its legitimacy. That means expending considerable resources to hold its territory in the face of superior conventional military forces.
Carrots and sticks
Gartenstein-Ross said al-Qaeda's strategy of waiting to declare a caliphate gave it more military flexibility to transition between a governing power and an insurgency force.
It doesn't help, he added, that ISIS has seemed to alienate its neighbours. This past February, a bombing that killed Abu Khalid al-Suri, a top commander of Syrian militant group Ahrar al-Sham, was widely believed to have been carried out by ISIS.
"ISIS does not play well with others. It has fought everybody. It fights the moderate rebels, it fights the extremist rebels. It fights everyone who’s not ISIS.”
However, the Centre for a New American Security's Heras is not as convinced.
"[ISIS] does want to dominate social structures in the areas that it's powerful, but it also understands that it has to employ carrots as well as sticks. To simply characterize it as a passing tide that will soon go back out to sea is, in my respectful opinion, to mischaracterize it. It is an organization that has sought to learn from the past and to learn from what didn't work in Iraq," he said.
Is there a possibility that al-Qaeda and ISIS might create a united jihadist front?
Stratfor's Stewart said al-Qaeda's highest levels "would like to reconcile" and bring ISIS back into their fold. But ISIS is probably less enthused about that idea.
"Once al-Baghdadi claims himself as the leader to all the faithful, it’s hard for him to make an oath of allegiance to anybody else on earth other than Allah."Suggest a correction