MARY LOUISE KELLY, HOST:

You could call it the Islamic State's last stand. Raqqa is the city in northeastern Syria that ISIS made its de facto capital more than three years ago. ISIS staged parades there and executions. Raqqa served as an operations hub, a terrorist training ground. It was the heart of the self-declared caliphate. Well, today, after four months of airstrikes and street-to-street fighting, U.S.-backed militias raised flags in the city and declared victory, prompting this question - what happens to ISIS now?

For an answer we turn to Joby Warrick, who covers national security for The Washington Post and who wrote a book about ISIS called "Black Flags," which won the Pulitzer Prize last year.

JOBY WARRICK: A lot of ISIS' followers around the world look at Raqqa as the most important capital. It's where the leaders were, at least for the longest time. It's where many of the big international operations, including the attack on Paris, were planned. It's where a lot of the things that we all identify with ISIS took place such as the beheadings. And for that city to be liberated and to have the last ISIS defenders driven out, that is a very powerful defeat for this organization and for its followers around the world.

KELLY: And we should note there are still pockets of resistance in that city. It may be some time before it's fully cleared. But the fighters who have fled, do we know where they're going?

WARRICK: In a million different directions. But some of them are going back to their hometowns. Some will be arrested and turned in. Others will just try to reorganize in the future. The one interesting thing we're seeing at this moment is that in some of these liberated cities, cities that were once held by ISIS, we're seeing an increase in terrorist attacks. So these groups are indeed able to kind of find their footing and find targets of opportunity and support that allows them to carry on terrorist attacks. And that will probably be the model going forward no matter what happens in Raqqa or Mosul or any of the big strongholds that we know about.

KELLY: So if the significance of today is the end of ISIS as a physical territory, the end of the Islamic State, what might that mean for ISIS the movement, ISIS the terror network?

WARRICK: So they are positioned to carry on the fight in at least a couple of ways. In the Middle East, including in Syria and Iraq, you know, there's a very good likelihood they just revert to being an underground terrorist network. This is what they know how to do. It's how they started. And they also know how to continue to exist as a virtual caliphate, as an idea that's promoted by a very powerful propaganda machine that can transmit messages and call for recruits around the world.

KELLY: Does the idea of ISIS lose a lot of its luster, though? I mean, they were able to point to this city and say, look. This is what we can do. This is what we can build. They've lost that today.

WARRICK: They have. And that is a big deal because as we examine the motivations of people that joined ISIS, foreign fighters over the last few years - and there have been tens of thousands of them - they were attracted to the idea of a physical caliphate, many of them, this place where they can go to and defend Islam against whatever perceived attacks they see from outside the region. It's also a place, you know, that had an army that seemed to be winning. And winning is part of this, too. You want to join a winning team. And you don't want to be part of a beaten organization. So a lot of that appeal is disappearing now that militarily they've been defeated.

KELLY: Last thing before I let you go, which is what does this mean for the U.S. strategy against ISIS? I mean, President Trump has framed the fight as a campaign to crush ISIS militarily. If ISIS has now been crushed militarily or something close to that, how does the U.S. see this fight going forward?

WARRICK: So there will be some bragging rights with the fall of the caliphate, and that's for sure. We've - you know, the Obama administration and Trump administration both put a lot of resources into militarily defeating this group. But what happens after - you know, when ISIS falls and then there's a terrorist attack in Paris again or maybe even in the United States? What's the response then? What, you know, easy targets are we going to go after in that case? And there will be attacks. I think it's virtually guaranteed that this group will continue to operate. It'll continue to do terrible things around the world. And it'll just be much harder for us to retaliate. And you can see the frustration that will inevitably arise because people won't really be sure what to do next.

KELLY: All right, thanks very much, Joby.

WARRICK: Always a pleasure, thank you.

KELLY: That's Joby Warrick of The Washington Post. He's also author of the book "Black Flags: The Rise Of ISIS." Transcript provided by NPR, Copyright NPR.