Here, too, the Islamic State’s successes have been objectively rare, in terms of absolute numbers. But the growing resonance between ISIS extremism and its anti-Muslim extremist counterparts—most recently embodied in Sunday’s attack on Muslims in London—is creating new risks for escalation. This means not just the possibility that each movement can attract new recruits, but also that their virulent ideologies could seize an ever-larger portion of the public discourse.

In a recent research paper for the International Centre for Counter-Terrorism—The Hague, I explored how identity collectives—such as nation, race, or religion—radicalize into extremism. I found that movements become more extreme when the legitimacy of the in-group, its right to exist as a collective identity, is challenged.

Real violence directed against members of an identity collective is the gravest imaginable challenge to the group’s legitimacy. Extremist ideologies are built from numerous sources, which can include scripture, history, and current events, ultimately boiled down to a collection of selectively chosen facts and outright fictions. Extremists do not require true information to undergird their ideologies, but when true information is available to support their claims, it strengthens their arguments and facilitates an increasingly violent orientation.

The symbiotic relationship between warring extremist groups provides extraordinarily potent grist for that mill. The violence ISIS carries out in the West provides fodder for anti-Muslim extremists, and growing violence against Muslims feeds back into the ISIS narrative. It is likely that this results in improved recruitment of extremist adherents, and it is almost certain that it results in a greater mobilization to violence among those who have already absorbed some portion of an extremist ideology.

When someone is attacked because of their religious, racial, or national identity, they will often question their place in society and the appropriate response. Years of research strongly suggest that extremist groups are particularly attractive to people who feel uncertain about their lives, their identities, and their roles in society.

The destabilizing and unexpected nature of terrorist violence contributes to these feelings of uncertainty, making the adoption of extreme identity-based ideologies attractive to more and more people. When two extremist movements are battling each other using innocent civilians as cannon fodder, the escalating violence and the toxic political rhetoric that follows serves to further amplify that uncertainty. The conflict creates situations that both sides can then incorporate into their own narratives. If the framing of the in-groups and out-groups sync up, the opposing ideologies can fit together like puzzle pieces, each completing the other’s argument about why the out-group is a threat.