ROBERT SIEGEL, HOST:
ISIS claimed responsibility for yesterday's deadly van attack in Barcelona. The group is losing ground on the battlefields of Syria and Iraq, but their terror attacks in European cities are increasingly frequent and successful. As NPR national security correspondent Greg Myre reports, these two developments are closely related.
GREG MYRE, BYLINE: The Islamic State offers detailed online instructions to supporters about how to carry out a vehicle attack - use a large truck, if possible. Avoid small cars that don't accelerate quickly. The list goes on and on. And ISIS supporters have been paying attention. The latest such attack in Barcelona was carried out by a speeding driver in a white van in a crowded part of the city at a time when it was filled with tourists.
DANIEL BYMAN: The attacks, to me, show both the strengths and weaknesses.
MYRE: Daniel Byman keeps tabs on terrorist groups as a professor at Georgetown University and a senior fellow at the Brookings Institution.
BYMAN: The strengths are obviously that it has an array of supporters, especially in Europe, that it can call upon to do attacks. The weakness, though, is that it has had difficulty doing more sophisticated operations.
MYRE: Case in point - most recent attacks have involved vehicles and knives rather than guns and explosives. Police believe a terrorist cell was attempting to put together a bomb on Wednesday at a house southwest of Barcelona. But it went off prematurely, destroying the house and killing one person, possibly the bomb maker. Apparently he wasn't working alone.
BYMAN: There seem to be multiple people involved. And thus it's kind of a broader network, a broader organization.
MYRE: The U.S. launched its air campaign against ISIS three years ago this month. As the Islamic State lost ground in Iraq and Syria, it unleashed attacks in Europe to show it was a potent force far beyond the Middle East. As its core territory shrinks, striking in Europe is now more important than ever. The group's most spectacular attack was the highly orchestrated assault in Paris that claimed 130 lives in November 2015. Since then, ISIS has also broken new ground by encouraging individual attacks by supporters who may have no formal links to the group.
JUAN ZARATE: I think ISIS and even al-Qaida at this point is applying an all-of-the-above strategy in terms of trying to get followers, and including those who have been trained, to use whatever tactics are available and possible.
MYRE: Juan Zarate was a deputy national security adviser under President George W. Bush and now runs a security consulting firm. He says Europe is and will remain vulnerable. ISIS has been very effective at radicalizing young European Muslims in their hometowns.
ZARATE: What you have are these ready-made jihadi networks in Europe amplified by what ISIS is able to direct and send back into the environment.
MYRE: Thousands of European Muslims have gone to the Middle East to fight for the Islamic State and could return home. This threat is guiding U.S. policy in the Middle East. The military doesn't want to just drive ISIS fighters out of group strongholds like Raqqa in Syria.
ZARATE: The idea is you cannot allow the escape of these fighters. You cannot allow some persistent safe haven because that threat will come back to haunt us.
MYRE: So the U.S. and its allies are encircling ISIS with the aim of crushing those fighters in Raqqa. Greg Myre, NPR News, Washington. Transcript provided by NPR, Copyright NPR.