The Islamic State claimed responsibility for a suicide bombing that killed 63 and wounded nearly 200 people at a wedding in Kabul over the weekend, signaling the group’s enduring reach and underscoring one of the challenges to a U.S. and Taliban peace negotiation.

The militant group lost its self-proclaimed caliphate in Iraq and Syria earlier this year. But Saturday’s bombing showed that the Islamic State remains a potent force beyond the borders it once claimed and fixed a glare on one of its lesser known but growing affiliates: the Islamic State in Khorasan, as the Afghanistan branch is known.

The bombing came as the U.S. and the Taliban work to finalize a peace deal that would end the 18-year war in Afghanistan. The Trump administration’s top negotiator presented a draft agreement to President Trump Friday that outlines a plan to withdraw thousands of American troops from the country — a move some fear would allow the Islamic State, also known as ISIS, to gain an even bigger footprint.

Here’s what is known about the Islamic State in Afghanistan:

The Islamic State has a growing foothold in Afghanistan

The Islamic State in Khorasan officially began operating in Afghanistan in 2015, according to a report by the Center for Strategic and International Studies. Its name invokes the Khorasan Province, a medieval region that encompassed parts of Afghanistan, Iran and Central Asia.

Started by Pakistani national Hafiz Zaeed Khan, who pledged allegiance to the Islamic State head Abu Bakr al-Baghdadi in 2014, it began as a small band of mostly Pakistani militants operating in the eastern Afghanistan province of Nangahar. Some recruits came from the Taliban, though members of other extremist groups in the region also defected to the Khorasan group, according to the CSIS report.

Like its parent group — the Islamic State in Iraq and Syria — the Afghanistan offshoot has ambitions to hold territory and is known for carrying out brutal attacks on civilians, including women and children. Shiites are particularly frequent targets.

The number of Islamic State affiliated fighters in Afghanistan has grown to include between 2,500 and 4,000, according to a recent estimate by the United Nations. They’re still mostly concentrated in Nangahar — but the wedding bombing in Kabul on Saturday highlighted the group’s growing reach, according to Michael Kugelman, a South Asia specialist at the Wilson Center.

Islamic State in Khorasan has never successfully captured territory in Afghanistan and it remains much weaker than rival militant groups in the region like the Taliban and al-Qaeda. Still, Kugelman said, the group has remained “resilient.”

Their strategy centers on attacking ‘soft targets’

The wedding attack was significantly larger and more destructive than other recent attacks, but it was “vintage ISIS in Afghanistan,” said Fawaz Gerges, the author of a recent book on the rise of the Islamic State. He noted that the group often attacks mosques, schools and weddings.

“The most effective means to stay in business is to carry out devastating attacks against soft targets, and to show that it’s purer than the Taliban and al-Qaeda,” said Gerges, who is also a professor at the London School of Economics.

The carnage produced by these sorts of attacks draws attention that keeps the group relevant, both in Afghanistan and across the world.

Since it was routed from Iraq and Syria, the Islamic State has increasingly turned its attention to other parts of the world, experts say — attracting recruits across Asia and Africa, and exporting its ideology and tactics as it seeks to transition from governing a caliphate to waging a global insurgency.

“Suicide attacks against weddings are force multipliers for ISIS, because it’s desperate to show its potency, its ability to strike near and far, especially after the beating it has taken in Iraq and Syria,” Gerges said.

Experts are divided over the threat ISIS poses in Afghanistan

Some analysts have speculated that Afghanistan could provide fertile ground for the central Islamic State organization to regroup. But experts and government officials disagree over just how great of a threat the group represents in Afghanistan.

Some believe it is an urgent danger: U.N. Secretary-General António Guterres warned this month that after losing its territory in Iraq and Syria, the umbrella group has access to hundreds of millions of dollars to finance terrorism and that Afghanistan is a fertile conflict zone from which to recruit fighters.

American military officers agree. They see the Islamic State in Afghanistan as a growing problem and worry that the local group could orchestrate attacks against the West, the New York Times reported. And Afghan generals say they have found fighting the group increasingly difficult.

But U.S. intelligence officials think the Islamic State in Khorasan lacks the know-how of the central group and does not pose a threat to Western countries.

Still, Gerges described the group as “a very stubborn affiliate."

“They have the potential to become a much more dangerous group,” he said.

The U.S. has fought the group in Afghanistan with mixed success

U.S. airstrikes took out key leaders of the Islamic State in Khorasan, including its founder, early on. And in 2017, the U.S. military dropped the “mother of all bombs” on a cave where fighters were hiding in Nangahar province. It was the first time the United States had used the most powerful bomb in its arsenal in combat.

Still, the affiliate group has managed to sustain itself and grow its membership.

“You’ve had a relentless campaign of Afghan and U.S. airstrikes in recent years … but despite all that, it’s remained there. It has not been dislodged; it’s continued to carry out attacks,” Kugelman said.

This has factored into ongoing peace talks between the Trump administration and the Taliban, particularly as the two parties wrestle over whether the United States should keep a counterterrorism force in the country after it withdraws other troops.

The latest draft of the peace deal, as described by U.S. officials, includes an initial withdrawal of about 5,000 of the 14,000 U.S. troops in Afghanistan in exchange for the Taliban renouncing al-Qaeda and working to shut down its operations in areas under Taliban control.

But the question of a long-term counterterrorism presence has proven to be a sticking point. The Taliban wants all U.S. forces out of the country.

So far, what to do about the Islamic State in Afghanistan hasn’t made it into the draft deal. Washington has previously said they want the Taliban to renounce the Islamic State, but the draft only mentions the group’s rival, al-Qaeda.

A peace deal could drive Taliban members to the Islamic State

The Taliban and the Islamic State are rivals and the Taliban has fought them in Afghanistan for years. The Islamic State’s ideology and tactics are significantly more extreme than those of the Taliban.

The Islamic State has sought to advance a global project through brutal means. In Afghanistan, it aims to foment sectarian violence by targeting Shiites, experts said.

The Taliban has also carried out attacks on civilian targets like schools and polling places. But unlike the Islamic State, the group denies deliberately targeting civilians. The Taliban sees the Islamic State as “an existential threat,” Gerges said, and it has increasingly sought to define itself in contrast to the Islamic State — as more reasonable, less bloody, and willing to operate as a mainstream political actor.

On Sunday, a Taliban spokesman took the extraordinary step of condemning the attack on Saturday as a “brutal act."

Despite the rivalry, disillusioned Taliban fighters have defected to Islamic State affiliates. Many of the group’s initial recruits formerly belonged to the Taliban.

A peace agreement with the United States could fuel another exodus. The Taliban is not a monolithic group, experts point out, and more extreme members may see dealing with the United States as a betrayal of the cause.

“If there is a Taliban peace deal, you would have a critical mass of disaffected, angry Taliban hard-liners who would jump into the ISIS camp,” Kugelman said.