For months, terrorism analysts warned that Islamic State-linked militants in Afghanistan would try to turn the Biden administration’s exit into a bloody spectacle.

On Thursday in Kabul, those predictions were realized.

ISIS-Khorasan, the Islamic State’s Afghanistan and Pakistan arm, issued a statement claiming responsibility for the suicide bombing attack that killed 13 U.S. service members and dozens of Afghans in an attack outside the airport. The series of blasts ripped through crowds of civilians who were clamoring for a chance to flee before the U.S. withdrawal deadline on Tuesday.

With its signature blend of complexity and cruelty, the attack was seen by many observers as a reminder to both the Americans and the Taliban that, no matter who was in the presidential palace, Afghanistan would remain contested.

Authorities instantly suspected the Islamic State affiliate, known as ISIS-K or ISK for short.

The group’s rivalry with the Taliban is a microcosm of the competition between al-Qaeda and its more radical spinoff, the Islamic State, analysts say. There are generational and doctrinal splits between the groups, with the Islamic State brand more popular with militants in recent years because it managed to capture territory and create a short-lived extremist fiefdom that spanned Iraq and Syria.

In Afghanistan, with the U.S.-backed government gone from power, ISIS-K can now focus on undermining its other local enemy, the Taliban, which analysts said will be hard-pressed to stave off attacks as it struggles to secure and govern a war-weary nation.

Amira Jadoon, an assistant professor at the U.S. Military Academy, has written extensively about ISIS-K, arguing that an unconditional U.S. withdrawal and Taliban takeover would bring about “the most permissive” environment for the group to operate.

“And this is what we are seeing now,” Jadoon said. “ISK’s main goal right now is to stay politically relevant, disrupt efforts to stabilize the country, and also undermine the Afghan Taliban’s credibility.”

In the hours before the attack, U.S. and Western governments warned of a specific threat related to ISIS-K and urged people to stay away from the airport. But the crowds returned Thursday, taking their chances before the evacuation window closed for good.

“An attack was said to be imminent and has now taken place,” tweeted Rita Katz, director of the SITE Intelligence Group, a terrorism-monitoring outfit that translated the ISIS-K statement. “Sadly, despite this intel, the attackers were still able to infiltrate.”

The 13 U.S. military deaths marked the first in Afghanistan since February 2020; the Pentagon said another 15 service members were wounded. They were days away from their homecoming and the effective end of the 20-year war.

Meanwhile, TV footage showed wounded Afghans writhing in pools of blood.

In a speech from the White House on Thursday evening, President Biden blamed the Islamic State and addressed the attackers directly: “We will not forgive, we will not forget, we will hunt you down and make you pay.”

Even before the Taliban takeover, U.S. officials were worried about ISIS-K’s spoiler role in Afghanistan’s transition. An Aug. 17 Defense Department report said, “ISIS-Khorasan exploited the political instability and rise in violence” from April through June “by attacking minority sectarian targets and infrastructure to spread fear and highlight the Afghan government’s inability to provide adequate security.”

“The blast at the airport today is showing that unfortunately a very bloody future is ahead of us,” said an Arab intelligence official, who spoke on the condition of anonymity because they were not authorized to be interviewed.

The official said the target of the attack was just as much the Taliban as it was the Americans: “It’s a battle over ideologies and hearts and minds.”

ROOTS IN PAKISTAN

ISIS-K began operating in Afghanistan in 2015, according to a report by the Center for Strategic and International Studies.

Started in 2014 by Pakistani national Hafiz Saeed Khan, who pledged allegiance to the Islamic State’s former head, Abu Bakr al-Baghdadi, it was 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.

As with other Islamic State affiliates, ISIS-K has stayed nimble and able to regroup after military blows. The founder died in a U.S. airstrike in 2016, according to CSIS. Dozens of fighters were killed in 2017, when the U.S. military dropped the “mother of all bombs” – the biggest one in the U.S. arsenal – on a cave the group was using as a hideout.

Still, the United Nations estimates that ISIS-K retains a core group of some 1,500 to 2,200 fighters in Afghanistan’s Konar and Nangahar provinces. Smaller cells are scattered across the country.

Attacks like the one at the airport are why armed groups are preparing for internecine battles, said an Afghan militant with ties to the Taliban and al-Qaeda, who spoke on the condition that he be identified only by a nom de guerre, “Abu Muhammad.”

“What happened today at Kabul airport is undermining the position of the Taliban, but they also know they can’t fight a war on two fronts,” Abu Muhammad said.

ISIS-K wants to destroy any coordination between the Taliban and the departing U.S. government and military, he said, adding that the suicide bombing was “a punishment also for all those who want to leave Afghanistan to go and live in the West.”

“The Taliban are trying to convince ISIS -K members to leave the group and rejoin the ranks of the Taliban, which might be difficult,” Abu Muhammad said. “We are all preparing for the possibility of a long war.”

Jadoon, the U.S. Military Academy academic, and co-author Abdul Sayed reported in March that ISIS-K had claimed responsibility for 47 attacks in the first two months of 2021, comparable to the group’s peak years of 2017 and 2018, when it was linked to 100 and 84 attacks, respectively.

Jadoon said ISIS-K is probably contemplating a number of ways to take advantage of the fluid situation in Afghanistan right now. She said three areas to watch are: the group’s efforts to recruit from the Taliban and other militant groups, prison-break operations to free fighters, and attempts to carve out strongholds in Nangahar and Konar.

The group is sure to continue targeting civilians in attacks that “tend to be more lethal and garner more publicity,” Jadoon said.

“Now without U.S. support or Afghan security forces,” she added, “I don’t think we can realistically expect the Taliban to constrain ISK without any additional support.”