As militants with the Islamic State of Iraq and Syria (ISIS) push further into Iraq, new scrutiny is being given to the threat posed by the growing number of Western fighters who have joined the ranks of the Sunni militant group.

The emergence of thousands of Western fighters inside of ISIS — fighters who hold foreign passports and require no visas to enter the United States — reflects an array of factors. Some security experts call it the result of an increasing generational divide between Al Qaeda and ISIS, while others attribute the expanding lure to the group’s stunning battlefield success, a savvy online recruitment strategy and the promise of life inside an Islamic caliphate.

Although precise figures are hard to verify, the number of Westerners now fighting alongside militants in Iraq and Syria has by all accounts surged. A June report by the New York-based intelligence organization The Soufan Group describes a region transformed into an “incubator for a new generation of terrorists,” with more than 12,000 foreign fighters from at least 81 countries stationed in Syria alone. Of that number, approximately 2,500 are from Western nations, including the United States, Australia, Belgium, Canada, France, Germany and Great Britain.

While the potential threat from these fighters is clear, what security experts continue to debate is how much of an immediate danger they pose to the U.S. and other Western targets.

Here’s why: Unlike Al Qaeda, which sees attacking the U.S. as key to its goal of establishing a caliphate, ISIS has functionally erased the border between Iraq and Syria and installed a virtual state of its own that now extends for hundreds of miles. The group’s advance spread deeper into northern Iraq last week, triggering a humanitarian crisis and prompting President Obama on Thursday to announce emergency aid missions and targeted airstrikes.

“Ideologically, they’re the same as Al Qaeda,” said Richard Barrett, a senior vice president at The Soufan Group. The difference for ISIS, he said, is that it’s leaders believe they must first “cleanse the area where the Muslim majority states are, and then having done that you can go out and beat up the rest of the world.”

It remains too early to tell whether U.S. airstrikes will push ISIS to recalculate, but so far the group has placed more of an emphasis on recruiting Western fighters to help administer their gains, rather than sending them home to carry out attacks.

“As long as they feel the West is trying to stay away … there is less motivation to try to attack,”  said Barak Mendelsohn, a professor of political science at Haverford College.

In June, the group issued a call for skilled professionals in its English-language periodical, Dabiq. In July a video released by the group’s media wing featured an English-speaking Canadian making a similar plea.

“This is more than just fighting,” said the man, identified as Abu Muslim. “We need the engineers, we need doctors, we need professionals … There is a role for everybody.”

“They have a problem not just of manpower in the frontline, but they have a problem of having enough technocrats really, enough people who can run the places they’ve conquered,” said Barrett.

To be sure, Western militants have also been heavily involved in the fight. In May, an American suicide bomber from Vero Beach, Fla., Moner Mohammad Abusalha, detonated 16 tons of explosives at a mountaintop restaurant in Syria. In a chilling video released after his death, he warns, “We are coming for you. Mark my words.”

That same month, two European ISIS members, one from France and the other from Denmark, died in twin suicide attacks in Iraq. According to an ISIS statement, the Frenchman “immigrated to al Sham [Syria] days after converting to Islam, and then to Iraq, seeking martyrdom in the cause of Allah.” The Danish bomber, ISIS said, drove a car packed with explosives into an Iraqi army convoy near the city of Mosul.

It is attacks such as these that have Western officials on alert. Speaking to ABC News last month, Attorney General Eric Holder said, “It’s something that gives us really extreme, extreme concern. In some ways, it’s more frightening than anything I think I’ve seen as attorney general.”

For now, experts are less worried about a large-scale operation originating from ISIS, than they are about attacks on scale with the May shooting that left three people dead at a Jewish museum in Brussels. The alleged shooter, Mehdi Nemmouche, was a 29-year-old Frenchman who had traveled to Syria in 2013 to join with extremists.

The attack, said Mendelsohn, raises the question of what happens when Western extremists return home from either Iraq or Syria disillusioned, potentially traumatized and intent to act on their own.

“You can imagine people that are back in their states, still unhappy, now even [more] indoctrinated … will decide independently to carry out some attacks.”

Echoing that sentiment, The Soufan Group noted, “Some of the foreign fighters may not return as terrorists to their respective countries, but all of them will have been exposed to an environment of sustained radicalization and violence with unknowable but worrying consequences.”

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