French police officers patrol near the Eiffel tower in Paris on Nov. 23. (Yoan Valat/EPA)

The terrorist attacks in Paris have reignited debate about the nature of the Islamic State and especially why so many young European Muslims have decided to join their cause. To explain the origins of armed mobilization, civil wars and terrorism, it is common to focus on material factors — the financial rewards or power that insurgents or terrorists would gain.

But our research suggests that two other factors matter as well: the emotions that inspire people to join armed movements and the ideologies that underlie their efforts. This is what we learned by closely studying an important case of armed mobilization: the Italian armed resistance movement against the Nazi-Fascist forces in World War II.

This armed movement was motivated by emotionally laden events. The Italian resistance was indignant about the policies of the Fascist regime — especially the human and material casualties that resulted from the decision to enter World War II. This pushed some Italians to reject the political status quo.

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At the same time, a radical ideology — in this case, communism — animated the Italian resistance. Communism provided the resistance movement with a specific group identity opposed to fascism and Nazism. Communist leaders helped articulate why it offered an alternative to the status quo. As one communist at that time put it:

We became members of the Communist Party because a Commissioner used to come and explain the motivations for political action.

However, not all ideologies led to armed mobilization. In 1940s Italy, other groups, like liberals and Christian democrats, did not advocate armed resistance. But ideology was important for those Italians that did take up arms against fascism.

It may seem strange to use a study of 1940s Italy to understand the Islamic State in 2015. But there are important implications from our research.

One is to take seriously the radical ideology espoused by the Islamic State. Of course, the Islamic State does not represent all believers of Islam. And  as a political theology, Islamism itself has changed in the last decades. However, the Islamic State’s theology is not just insignificant rhetoric.

As Olivier Roy has argued, many of those that join jihad adopt a specific version of Islam, Salafism, because it provides a rigid worldview with clear guidance about what is wrong and what is right. This form of radical ideology easily translates norms into actions. Iyad el-Baghdadi puts it even more clearly: “Rules give structure, and they give meaning. In the midst of all of this chaos around you, there are these rules, and they’re defined rules and they make sense.”

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A second implication is that members of the Islamic State — although more diverse than the Italian resistance — are acting based on some emotional triggers. It could be indignation, it could be resentment or it could be despair.

For instance, during the recent and very heated British parliamentary debate preceding the vote on Syria airstrikes, some speakers stressed how the killing of innocent civilians could cause a strong emotional reaction among the Muslim community and, instead of weakening the Islamic State, increase the number of foreign fighters. This argument was also put forth two months ago by the largest British Islamic organization when discussing and criticizing drone attacks in Pakistan.

In sum, as much as material interests matter, factors like emotions and ideology can be crucial in explaining violent mobilization and radicalization. Their combination can be particularly lethal.

Stefano Costalli is Isaac Newton Fellow and member of the Michael Nicholson Center for Conflict and Cooperation in the department of government at the University of Essex.

Andrea Ruggeri is associate professor of international relations in the department of politics and international relations at the University of Oxford and a research fellow of the Michael Nicholson Center for Conflict and Cooperation, in the department of government at the University of Essex.