The battle to retake Raqqa, the capital of ISIS’s self-proclaimed caliphate, has officially begun. And US officials are confidently saying that it will be a bloodbath — for ISIS.

“The mission is to make sure ... that any foreign fighter who is here, who joined ISIS from a foreign country, who came into Syria, they will die here in Syria,” Brett McGurk, the president’s special envoy to the fight against ISIS, told journalist Jenan Moussa during an interview. “If they are in Raqqa they are going to die in Raqqa.”

Moussa, a journalist for the Dubai-based Arabic news channel Al Aan TV, caught up with McGurk in the northern Syrian city of Ain Issa, about 30 miles north of Raqqa. McGurk was there visiting the offices of the Raqqa Civil Council, the local council set to take over the running of Raqqa once ISIS is defeated.

The offices, located next to an ISIS building that had been destroyed by airstrikes, was an unusual place for an interview; policymakers as senior as McGurk rarely travel so close to the front lines of an active war zone. As Moussa noted on Twitter, “This is symbolic. A while ago ISIS was ruling this place. Now I'm driving around freely & having an interview with the U.S envoy.”

Analysts have long warned of the potential terrorist threat posed by foreign fighters that flocked to Syria and Iraq — particularly those from Western Europe — returning to their home countries as ISIS’s territory shrinks. Up to 30,000 foreign fighters are believed to have traveled to Syria to join ISIS, but the US government estimates that as many as 25,000 of them have since been killed. Back in March, the Pentagon estimated that between 3,000 and 3,500 ISIS fighters were still inside Raqqa, many of them foreign fighters.

“If they are in Raqqa they are going to die in Raqqa”

And though some ISIS fighters, and many of its leaders, have likely fled the city since then in advance of the coming onslaught, there is certain to be a hardcore cadre of fighters who have stayed behind to fight. Raqqa is the heart of ISIS’s territorial holdings and has been home to some of the group’s most prominent leaders since ISIS conquered the city in January 2014. Many of those militants are likely to be willing to fight to the death there.

“[ISIS] has been fortifying its positions in Raqqa for months, setting up barriers and hanging sheets of cloth over main streets to provide cover from warplanes,” the Syrian Observatory for Human Rights, a pro-opposition monitoring organization based in the UK, reports. “A belt of land mines and militant checkpoints circle the city.”

That means the US, and its allies on the ground, will soon find out just how hard ISIS is willing to fight — and just how high a price will be paid to retake the city.