The group known as ISIS-K had long planned attacks on American personnel and others. That's one reason why President Biden said he wanted to limit the duration of the U.S. withdrawal from Afghanistan.
On Thursday, this regional affiliate of the Islamic State struck at the heart of Kabul, setting off an explosion outside Hamid Karzai International Airport. The attack killed more than 150 Afghan civilians and at least 13 U.S. service members.
The next day, the U.S. hit back with a drone strike that killed two ISIS-K targets involved in the airport attack, the Pentagon said.
Here's what is known about the group, which reportedly has claimed responsibility for Thursday's bombing:
What is the Islamic State affiliate in Afghanistan?
The Islamic State Khorasan emerged more than six years ago and operates as an ISIS affiliate in Afghanistan and Pakistan. Khorasan is a historical term for a region that includes present-day Afghanistan, Pakistan and surrounding countries. The group is also known as ISIS-K, ISK or ISKP.
The founding members included militants who left both the Afghan Taliban and the Pakistani Taliban.
"ISIS had sent representatives to both Pakistan and Afghanistan. They were essentially able to co-opt some disaffected Pakistani Taliban and a few Afghan Taliban [members] to join their cause," Seth Jones, an Afghanistan specialist at the Center for Strategic and International Studies, said on NPR's All Things Considered.
In a 2015 video, the group's leader at the time, Hafiz Saeed Khan, and other top commanders pledged their allegiance to Abu Bakr al-Baghdadi, then the Islamic State's leader, and declared themselves administrators of a new ISIS territory in Afghanistan. (While some research reports put ISIS-K's founding date in 2015, others say it emerged in 2014.)
The regional affiliate governed with a strict interpretation of Islamic law and used violent enforcement tactics, such as carrying out public executions, killing tribal elders and closing schools, according to the Center for International Security and Cooperation at Stanford University.
How is the Islamic State group tied to the Taliban?
The two are actually enemies, as Biden noted in his televised address Thursday. Since its founding, the Islamic State affiliate has been at odds with the Taliban, which now control Afghanistan.
"Their goal really is an Islamic emirate, and they are a competitor of both al-Qaida and the Taliban," said Jones of the Center for Strategic and International Studies.
Many Taliban militants defected to join the Islamic State affiliate, and the two groups fight for resources and territory.
Their differences are also ideological, according to the Stanford center.
"The hostility between the two groups arose both from ideological differences and competition for resources. IS accused the Taliban of drawing its legitimacy from a narrow ethnic and nationalistic base, rather than a universal Islamic creed," the center said.
As The Associated Press has reported, as the Taliban sought to negotiate with the United States in recent years, many of those opposed to talks switched over to the more extremist Islamic State.
The Taliban condemned the blasts outside the airport Thursday and said the U.S. controlled the area where the attacks occurred.
Biden turned the focus back to the Taliban on Thursday, saying, "It is in the interest of the Taliban that ISIS-K does not metastasize."
How big of a threat is ISIS-K in Afghanistan?
As of 2017, the U.S. military estimated that it had killed 75% of the Islamic State affiliate's fighters, including some of its top leaders.
The Center for Strategic and International Studies counted almost 100 attacks by the group in Afghanistan and Pakistan by 2018, and hundreds of clashes with U.S., Afghan or Pakistani forces.
Monitors of U.N. sanctions believe the affiliate has around 2,000 fighters in eastern and northern Afghanistan but also noted that the group has had to "decentralize" after significant territorial losses.
But according to the Congressional Research Service, the group has claimed responsibility for a string of high-profile attacks, including the May bombing of a girls' school in Kabul.
In a Pentagon briefing following the attack outside the Kabul airport, Gen. Kenneth McKenzie, the head of U.S. Central Command, said that "the threat from ISIS is extremely real" and that there were other active threats against the airfield in Kabul.
The attack also could reveal holes in the Taliban's abilities. "What this does show, by the way, is that Taliban's counterintelligence and counterterrorism capabilities actually are somewhat limited," Jones said on All Things Considered. "They were not able to identify or stop the attack."
MARY LOUISE KELLY, HOST:
OK. The terrorist group Islamic State Khorasan or ISIS-K has claimed responsibility for these attacks at the airport. President Biden spoke earlier this month about the threat this group poses.
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PRESIDENT JOE BIDEN: Every day we're on the ground is another day we know that ISIS-K is seeking to target the airport and attack both U.S. and allied forces and innocent civilians.
KELLY: So who and what is ISIS-K? Seth Jones is an Afghanistan specialist at the Center for Strategic and International Studies. He's with us now.
SETH JONES: Hi. Good to be on.
KELLY: I suspect a lot of Americans may not have heard of ISIS-K up to now. Give us just briefly the description - where did they come from? What do they believe?
JONES: Well, ISIS-K was formed around the fall of 2014. ISIS had sent representatives to both Pakistan and Afghanistan. They were essentially able to co-opt some disaffected Pakistan Taliban and a few Afghan Taliban to join their cause. Remember; this was around the time of the expansion of the Islamic State in Iraq and Syria. So they have continued to operate. They grew in numbers for a while. They've come down. But their goal, really, is an Islamic emirate. And they are a competitor of both al-Qaida and the Taliban.
KELLY: And how do they fit into the complex picture that is Afghanistan? I mean, elaborate on what you just said, that they're enemies of the Taliban.
JONES: They're enemies of the Taliban. The Islamic State use Afghanistan as an important part of its global jihad. But as we've seen in Iraq and Syria and other locations across the globe, they view al-Qaida as a major competitor. And in Afghanistan, al-Qaida has made a decision to embed within Taliban forces on the ground. So the Islamic State has taken a very different tack. Instead of embedding forces, they've decided that they're going to conduct high-profile attacks, something al-Qaida has not done. So they've taken on a much more public dimension and conducted a range of attacks, obviously now targeting U.S. military service members as well as Afghan civilians.
KELLY: And what, I guess, up to today, did we know about their capabilities? Would you have thought they were capable of pulling off an attack like this?
JONES: Absolutely. What the Islamic State has shown is a willingness to conduct high-profile attacks in Afghanistan. Their numbers have dwindled to about 2,000 from probably 6- or 7,000 a couple of years ago, but they've had active cells across the country, including in Kabul itself, and willing to conduct high-profile attacks. What this does show, by the way, is that Taliban's counterintelligence and counterterrorism capabilities actually are somewhat limited. They were not able to identify or stop the attack. They made their way through Taliban checkpoints to get to the airport.
KELLY: And tell me a little bit more about the relationship between ISIS-K in Afghanistan and core ISIS, let's call it, the group we know so much about from Iraq and Syria.
JONES: Well, core ISIS started ISIS-K by deploying a number of individuals into the Afghan-Pakistan area again in late 2014. But I would say, generally, since the collapse of ISIS in Iraq and Syria, the organization has largely operated independently. They're run right now around the Afghan province of Kunar, but they have cell structures that they operate largely autonomously from ISIS in Iraq and Syria. So they're connected in some ways as a branch of broad ISIS, but they operate largely independently on a day-to-day basis.
KELLY: And just briefly, big picture, I'm struck - part of the White House rationale for getting out of Afghanistan was it was, allegedly, no longer a safe haven for terrorists who could attack the U.S. And here we see today, in the very last hours of U.S. boots on the ground, terrorists in Afghanistan attacking U.S. troops.
JONES: Well, the great irony of the U.S. withdrawal from Afghanistan is that while President Biden said last Monday that it was terrorism that was really the only remaining interest and the terrorism threat had declined, it is - day by day, it is now getting worse. And I think the other aspect we saw is the release of prisoners of al-Qaida and the Islamic State that's also part of the problem.
KELLY: Who are now joining the ranks. Absolutely. We will leave it there. Seth Jones of the Center for Strategic and International Studies.
JONES: Thank you. Transcript provided by NPR, Copyright NPR.