Pape’s theory is that suicide terrorism is fundamentally a response to military intervention—in the form of a rival occupying territory that the terrorists prize. For “nationalist” reasons, the terrorists want to control that territory, as any state would, through a monopoly on force and exclusive political authority. The argument here isn’t that all territorial occupations produce suicide terrorism, or that every individual terrorist is chiefly concerned with contested land, but rather that terrorist groups that today practice suicide terrorism tend to be grappling with dynamic losses of territory. Drawing on a database of suicide attacks around the world since 1982, Pape claims that his geopolitical paradigm has more predictive power than, say, explanations for terrorism that focus on religious fanaticism.

The idea that ISIS is primarily driven by extreme Islamist ideology suggests that “the targeting logic of a group comes right from its [religious] doctrine,” Pape told me. “Given that ISIS’s doctrine has not changed—that is, it’s still a religious group—then there should never have been a shift of its targeting tactics.” And yet a shift in who it targets seems to have occurred. Why?

In Pape’s view, ISIS has trained its sights on countries like Belgium, France, Russia, and Turkey because the U.S. coalition’s air and ground campaign, along with military operations by Russia and its ally, Syrian President Bashar al-Assad, have significantly eroded ISIS territory in Syria and Iraq in recent months (Belgium has participated in air strikes against ISIS in Iraq, but not Syria).

According to one recent estimate by IHS Jane’s 360, the Islamic State lost control of 14 percent of its territory between January and mid-December 2015, and an additional 8 percent in the last three months (in the map below, red represents losses, green gains, gray no change).

ISIS Territorial Gains and Losses: January 1, 2015 – March 14, 2016

In other words: In response to Ted Cruz’s statement on Tuesday, following the bloodshed in Brussels, that “radical Islam is at war with us,” Pape might agree that ISIS is a radical Islamist group. But he likely wouldn’t agree that the precepts of radical Islam are determining the course of the war that ISIS is waging.

“The ebbs and flows of territory are predictive of the group’s targeting logic,” Pape told me, and the evolution of that logic over the last six months might be the key lesson from the Brussels attacks, even if the violence may have more proximate causes as well, such as the arrest last week in Brussels of one of the plotters of the Paris attacks. “ISIS is now losing in Iraq and Syria—they’re losing actually quite badly—and so they’re now in a position where they’re trying to change a losing game,” he said. The less in control the organization is at home, the more it strikes at targets abroad.