Since declaring a caliphate, Islamic State has garnered support from more than a dozen Islamist militant groups in the Middle East and Asia, but the dearth of endorsements by many of the largest and most recognizable groups serves to underscore the limits of the newcomer's grand ambition.
The Al Qaeda breakaway group has proved a lightning rod for devout supporters and bitter enemies since entering the Syrian civil war in April 2013. Its sweeping advances into Iraq, gruesome tactics and, most pointedly, its declaration of an ultra-conservative Sunni Muslim state covering the wide portions of eastern Syria and northern Iraq that it now controls have led both to U.S.-led airstrikes and pledges of allegiance by 13 fellow Islamist militant groups.
Others have thrown their support behind Islamic State without placing themselves under the leadership of self-proclaimed caliph Abu Bakr Baghdadi.
But the best-known Islamist militant networks — such as major Al Qaeda affiliates the Shabab and Al Qaeda in the Islamic Maghreb in Africa, the Taliban in Pakistan and Afghanistan, and Boko Haram in Nigeria — have not signed on to the movement that is seeking to position itself as the rightful leader of Muslims worldwide and the preferred destination for would-be Islamist fighters.
"These elements of support do exist, but I don't think the caliphate announcement was as galvanizing or caused the huge shift that ISIS hoped it would," said Aymenn Jawad Tamimi, a fellow at the Global Research in International Affairs Center in Israel, using a common acronym for Islamic State.
The closest the group has gotten to high-level backing was a recent statement by the Yemen-based Al Qaeda in the Arabian Peninsula affirming support "for our brothers against the global Crusader campaign." But the ideological differences between the groups is probably too great for this to lead to anything beyond solidarity.
Though the bump from its caliphate declaration may not have been as large as the group hoped, Islamic State's popularity among militants has drawn more into its fold.
"That's one trump card they have, and you see that a lot on their recruitment messaging; ISIS is still emphasizing that most foreign fighters coming to Iraq and Syria are joining them," Tamimi said.
Indeed, the tactic has the feel of a marketing strategy, coming from a group that has exhibited social media savvy. Thus far, though, they have scored the militant equivalent of D-list celebrity endorsements.
The Somali-based Shabab recently rejected an attempt by Islamic State representatives to buy its allegiance, according to a report by the U.S.-based SITE Intelligence Group, which monitors online militant activity.
"The majority of groups pledging support are smaller, lesser known groups that have weak or no ties to Al Qaeda central leadership, and are looking to affiliate themselves with [Islamic State] in order to bolster their own jihadist credentials," Evan Jendruck, a terrorism analyst at Jane's Terrorism and Insurgency Centre, said in an email.
For these mostly regionally focused groups, pledging to Islamic State has marked their first entry to the fray of global insurgency.
Islamic State's high media profile and continued momentum on the battlefield in Iraq and Syria has served to attract "smaller factions around the world looking for a parent organization to tap into," said Charles Lister, a visiting fellow at the Brookings Doha Center.
Those that have shown their support — if not complete loyalty — by funneling fighters to Islamic State include militant groups in Tunisia and the Gaza Strip. It remains to be seen what consequences this loyalty could have for Islamist militant causes beyond Iraq and Syria and whether Islamic State's brutality will spread.
"Beyond attention-grabbing headlines, actual insurgent conflicts remain largely unchanged, so far," Lister said.
In a speech in September, Islamic State spokesman Abu Muhammad Adnani called on Muslims to attack Westerners and specifically to "strike the soldiers, patrons, and troops of the tyrants. Strike their police, security and intelligence members, as well as their treacherous agents."
That same month, a French tourist was beheaded in Algeria by an Islamic State-linked group called Jund Khilafah, which had first warned that it would execute him within 48 hours unless France stopped airstrikes in Iraq.
In the video of the beheading, one of the killers said, "This is why the soldiers of the caliphate in Algeria have decided to punish France, by executing this man, and to defend our beloved Islamic State." It echoed the videos released by Islamic State in which American and British journalists and aid workers were beheaded in retaliation for airstrikes.
In the Philippines, the militant group Abu Sayyaf, which has pledged allegiance to Islamic State, threatened to kill two German hostages before releasing them on Oct. 17. The group had demanded that Berlin pull its support for the U.S.-led coalition and pay a $5.6-million ransom. Abu Sayyaf's spokesman said the ransom was paid.
Unlike Al Qaeda's central command, which has affiliates in several global hot spots, as well as more casual supporters, Islamic State demands nothing less than absolute fealty. That has alienated some potential supporters, who view Islamic State as an interloper that has risen to the top too quickly.
"There are a larger number of groups who are pledging affiliation to Al Qaeda to align themselves against ISIS," said Thomas Lynch III, a research fellow at the National Defense University's Institute for National Strategic Studies in Washington, D.C.
Islamic State has disregarded established covert methods by recruiting openly and indiscriminately, thus failing to learn the lessons of predecessors and providing Western intelligence units the ability to track communications sites and fighter locations through social media, Lynch said.
It has also openly threatened countries around its home territory, actions that other Islamist militants see as rash and careless. "The serious jihadi outfits and networks are really mobilizing against ISIS, who are seen in the jihadi space as a usurper," Lynch said.
Yet Islamic State continues to win support, even causing fractures in some groups.
Top officials, including the official spokesman, of the Pakistani Taliban — known as Tehreek-e-Taliban Pakistan or TTP — have pledged to Islamic State. The leader of TTP, an umbrella group of local and Al Qaeda-affiliated militants, has not done the same.
In Syria, U.S.-led airstrikes aimed at debilitating and defeating Islamic State have had the opposite effect on the group's popular backing. From Dair Alzour in the east to Aleppo in the northwest, residents and religious leaders have in part rallied behind Islamic State, which until recently was reviled and feared because of its brutal interpretation and implementation of Islam.
In Aleppo province, where the Western-backed Free Syrian Army and Islamic Front have controlled large chunks of territory, imams have taken to the pulpit in favor of Islamic State, speaking of the airstrikes as "a crusader war on Muslims," said Humam Halabi, a member of the Al Qaeda-affiliated Al Nusra Front.
The rise in ground support has corresponded with an exodus of fighters from other groups — especially foreign fighters — joining Islamic State's ranks.