What Happens When ISIS Goes Underground?

A view of Islamic State slogans painted along the walls of a tunnel near Mosul, Iraq, March 4, 2017. Reuters/Alaa Al-Marjani

Defeating the Islamic State could be the marquee foreign-policy accomplishment of the Trump administration. Doing so, however, will require more than just forcing the caliphate underground.

January-February 2018

The Islamic State will likely continue, and may even focus on, terrorist attacks in the West. In 2015, Paris suffered the worst terrorist attack on French soil in history when Islamic State gunmen and suicide bombers killed 130 people. France and other European states saw smaller attacks that year and the next, and by October 2017, five attacks linked to or inspired by the Islamic State targeted the United Kingdom—its most lethal year in terrorism since 2005. These high-profile attacks allow the Islamic State to maintain relevance to would-be and current supporters, convincing them to fight for the group despite its setbacks in Iraq and Syria. The attacks also signify revenge for the group’s tremendous losses. Although U.S. efforts to destroy the Islamic State’s havens hinder the group’s ability to carry out sophisticated attacks, some attacks involve an Islamic State facilitator who helps recruit or directs the attacker, but does not provide elaborate operational support. Many of these attacks are low-tech but quite bloody: the Bastille Day attacker in Nice in 2016, for example, killed eighty-six people by driving a truck through a crowd.

Internal dynamics make Europe a particularly likely target, and in the short term the terrorism threat may grow as the caliphate collapses. The Syrian conflict has attracted over six thousand European volunteers. Some of these European foreign fighters will die and some will stay in the war zone, but some will also likely return to their home countries. One EU official estimates that approximately 1,500 will return. A fraction of those who return home may commit terrorist attacks or recruit locals to join the cause. The potential size of that fraction is unclear, but even a small percentage out of 1,500 can frustrate local police and security services. Europe contains more radicalized Muslims relative to their overall population, as suggested by the dramatically higher number of foreign fighters from European states. In addition, many European Muslims integrate poorly into their broader communities, which discourages them from cooperating with local intelligence and law-enforcement services. Finally, European intelligence services vary in skill: some, including those of France and the United Kingdom, are highly skilled, while others, such as Belgium’s, are under-resourced and less capable of responding to terrorism threats. Fortunately, with heavy U.S. prodding and support, European states have improved intelligence cooperation and otherwise tightened their defenses. But this will remain a long-term challenge.

IN COMPARISON with Europe, the Islamic State poses a more manageable threat to the U.S. homeland. Since the September 11 attacks, ninety-seven Americans have died in jihadist-related attacks in the United States (the figure was ninety-five until the October 2017 truck-ramming attack in New York City, which killed two Americans and six foreign visitors). The two deadliest attacks, in San Bernardino in 2015 and in Orlando in 2016, that together killed sixty-three Americans, involved individuals who claimed some allegiance to the Islamic State but acted independently of the group—often referred to as “lone wolves.” Although any death from terrorism is deplorable, the number of American deaths in the U.S. homeland—ninety-seven—is far lower than many experts, both inside and outside of government, predicted.

Multiple factors likely explain this relatively low level of violence. First, senior U.S. officials overestimated the number of radicals in the United States after 9/11 when they spoke of thousands of jihadist terrorists in the United States. Second, the American Muslim community regularly works with law enforcement, leading to many arrests. As former FBI director James Comey explained,

“They do not want people committing violence, either in their community or in the name of their faith, and so some of our most productive relationships are with people who see things and tell us things who happen to be Muslim.”