Of course, ISIS also relies on more traditional methods of recruiting. It appeals to Muslim fundamentalists by citing historical references; by claiming to include descendants of pious families of ancient, respected lineage and stature; and by identifying its leaders as the messengers and executors of apocalyptic prophecies. All of these themes have a very powerful effect on Muslim communities familiar with these stories and traditions. Its eschatological messages in particular, as ISIS expert Will McCants recently wrote, have greatly “invigorated” the group, accounting for “the inrush of foreign fighters to Syria, many of them seeking a role in the End-Time drama.”

One does not have to speculate terribly much to see the potential threat from ISIS to the West given its vast cadre of foreign fighters native to, or previously resident in, those countries. This unprecedented pool of foreign recruits suggests that ISIS would certainly have the capability to undertake more attacks modeled on the simultaneous assaults and running gun battles that occurred in Mumbai in November 2008 and Paris almost exactly seven years later. Given that ISIS has already established clandestine logistical support bases in Turkey, the prospect of some of its fighters piling into a car with a trunkful of Kalashnikov assault rifles, hand grenades and RPGs and driving into the European Union undetected is not outside the realm of possibility. It is also not difficult to imagine a less complex operation—such as the one mounted by the Somali jihadi terrorist group, al-Shabab, in September 2013 that targeted Nairobi’s Westgate shopping mall—occurring in some European city or suburb. Either type of attack would be well within the grasp and capabilities of ISIS fighters in Europe or even in North America, requiring little in the way of resources or planning.

Any notion that this potential has not already been pursued by the Islamist terrorist organizations operating in Syria was put to rest by the American journalist, Theo Padnos, who spent two years in Syria as a hostage of ISIS’s rival, the Nusra Front. In his account published last year in the New York Times, Padnos related how

The Nusra Front higher-ups were inviting Westerners to the jihad in Syria not so much because they needed more foot soldiers—they didn’t—but because they want to teach the Westerners to take the struggle into every neighborhood and subway station back home.

Given the symmetry between ISIS’s approach and Al Qaeda’s, a Mumbai-style type of attack in Europe or elsewhere in the West would also have the attraction of fulfilling bin Laden’s 2010 wish to stage these type of assaults across Europe. Nor can attacks along the lines of the July 7, 2005, London suicide bombing on mass transit be ruled out. Similar plots have already been thwarted at least twice in recent years—in Barcelona in January 2008 and New York City in September 2009. Conventional bomb attacks against mass transit that do not involve suicide bombers are indeed also possible—much as occurred in Madrid in March 2004 and in Mumbai two years before the November 2008 simultaneous assaults that galvanized world attention.

In addition, ISIS-trained fighters might not return home empty handed. The threat of a chemical-weapons attack targeting civilians elsewhere in the Middle East or in Europe has to be considered in respect of ISIS’s repeated use of these agents in Syria, often against civilian populations. There are already indications of ISIS’s interest in a variety of harmful toxins for use as weapons. For example, in May 2013 Iraqi authorities arrested an ISIS cell in Baghdad overseeing the production at two factories of sarin and mustard blistering agents.

Finally, there is the prospect of a grand bargain and reconciliation between ISIS and Al Qaeda, which would profoundly change the current conflict and result in a significantly escalated threat of foreign-fighter terrorist operations in the West. It was only a little more than a year ago that the conventional wisdom inside the Beltway was that the bloody split between Al Qaeda and ISIS would neuter and ultimately destroy them both. As the conventional wisdom on Al Qaeda has rarely been correct anyway, it is not surprising that this too proved to be little more than wishful thinking. Indeed, efforts to reunite are continuous from both sides—as al Zawahiri alluded to in a recent public statement. Significantly, ISIS propaganda has always been deeply respectful of Al Qaeda, referring to its soldiers, emirs and sheikhs in a positive manner and continuing to glorify bin Laden’s accomplishments. The main impediment to reconciliation, however, is the strong personal enmity and vicious rivalry between al Baghdadi and al Zawahiri. Hence, al Baghdadi’s death would likely pave the way for a rapprochement producing a combined terrorist force unprecedented in scope, size, ambition and resources.

Fifty-four years ago, Jean-Paul Sartre warned of the “boomerang” effect in his preface to Frantz Fanon’s The Wretched of the Earth. “It is the age of the boomerang,” he observed, a new phase “of violence: it flies right back at us, it strikes us and, once again, we have no idea what hit us.” More than half a century later we are again confronted by the boomerang—this time from a hard core of battle-hardened terrorists drawn from the thousands of foreign fighters trained and commanded by ISIS who may at this very moment be awaiting orders to deploy back to their homelands.

Bruce Hoffman directs Georgetown University’s Security Studies Program and is a senior fellow at the U.S. Military Academy’s Combating Terrorism Center. His latest book is Anonymous Soldiers: The Struggle for Israel, 1917-1947 (Knopf, 2015).

Image: Wikimedia Commons/Voice of America