The “hardcore fighters,” especially the foreign ones within the inner circle of ISIS leader Abu Bakr al-Baghdadi and his top commanders, will likely remain in Iraq and Syria, and look to join the underground resistance of an “ISIS, 2.0.” In all likelihood, these guerrilla insurgent shards of ISIS will congeal into a clandestine terrorist organization. Besides conducting sporadic raids, ambushes, and, perhaps, spectacular attacks using suicide tactics, these ISIS fighters will rest, rearm, and recuperate.
During this time, the militants may switch their allegiances between a smattering of groups on the ground, including ISIS, Jabhat Fateh al-Sham, and Ahrar al-Sham (already a loose coalition of Islamist and Salafist units), and will actively seek out ungoverned areas still beyond the writ of either Syrian or Iraqi government forces and their allies. As the terrorism expert Bruce Hoffman has suggested, if the fortunes of ISIS continue to decline, there may be a group of jihadists that see rapprochement with al-Qaeda as the only option to continue their struggle. Interviews with some Western ISIS fighters suggest that the ideological differences between al-Qaeda and ISIS are too significant to be bridged quickly, but this may change over time.
A second group of fighters are those potential “free agents” or mercenaries who are prevented from returning to their home countries. They can be expected to form a cohort of stateless jihadists who will travel abroad in search of the the next jihadi theater—Yemen, Libya, West Africa, or Afghanistan—to protect, sustain, and expand the boundaries of the so-called caliphate. These are the militant progeny of the original mujahideen, or transnational jihadists that once filled the ranks of al-Qaeda and fought in Afghanistan against the Soviets, and in Chechnya and the Balkans. ISIS affiliates and local Sunni jihadists in these places would likely welcome an influx of battle-hardened comrades.
And then there is the third group of foreign fighters: “the returnees.” This is the cohort that most concerns those in counterterrorism circles. These fighters may attempt to return to their countries of origin, like Tunisia or Saudi Arabia, or go further afield to Europe, Asia, or North America. States with more robust national defense structures—well-trained border police, world-class intelligence services—stand a better chance of blunting their impact. But all Western security services are not created equal: Some will inevitably have a tougher time containing this threat than others. Further complicating the issue is the inability among nation-states, especially those within the European Union, to even agree on the definition of “foreign fighter.”
The returnees are not as homogeneous a group as they may seem. Some will be among the “disillusioned”—those that went to Syria in search of utopia, adventure, and an opportunity to express their religious identity and instead found something far different. According to interviews and other research, local Syrians—whom the fighters said they went to “save”—did not respect them. These fighters struggled to fend for themselves when it came to obtaining basic things like food and financing, and grappled with the tribulations of war. But upon returning to the West, they could be used to mentor other radicalized youth. These fighters may require psychological care, not prison time.