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What is ISIS-K? Here’s what the Taliban takeover means for al-Qaeda and the Islamic State’s Afghanistan affiliate.

Anti-Taliban fighters watch explosions from U.S. bombings in the Tora Bora mountains in Afghanistan during the hunt for al-Qaeda fighters in December 2001. (Erik De Castro/Reuters)

U.S. officials blamed an Islamic State affiliate in Afghanistan for twin blasts outside Kabul’s airport Thursday, and President Biden pledged to track down leaders of the group who ordered the attack.

The explosions left dozens dead or injured, including U.S. military personnel and Afghan civilians. Crowds have thronged the airport since the Taliban took over Kabul on Aug. 15, and U.S. intelligence officials had anticipated the Islamic State Khorasan (ISIS-K) could strike amid the chaos.

The Islamic State released a statement Thursday claiming responsibility for the attack.

The ISIS offshoot is a more extreme rival of the Taliban. Experts say the Taliban is likely to try to root out the group. But some warned earlier this month that ISIS-K could benefit from a security vacuum as the Taliban tries to consolidate power.

The U.S. withdrawal was based on the conclusion that terrorist groups would no longer be able to use the country to stage attacks on the United States.

“We went to Afghanistan almost 20 years ago with clear goals — get those who attacked us on September 11th, 2001, and make sure al-Qaeda could not use Afghanistan as a base from which to attack us again,” President Biden said in remarks from the White House last week defending the pullout of American forces after the Afghan government’s swift collapse. “We did that.”

But the bloody scenes outside the airport Thursday have underscored the ongoing terrorism threat in Afghanistan, where al-Qaeda also continues to operate.

While al-Qaeda has been substantially weakened since 2001 — and the Taliban has committed to preventing it from attacking the United States and its allies — al-Qaeda fighters remain in Afghanistan and have hailed the Taliban takeover.

The Taliban has said it would not allow al-Qaeda or other terrorist groups to launch attacks from the country. But outside observers say the Taliban maintains ties to al-Qaeda.

Here’s where the Islamic State and al-Qaeda stand in Afghanistan.

Taliban insists it will not shelter al-Qaeda in Afghanistan this time around

Ireland's representative to the United Nations urged the Security Council on Aug. 19 to reject the Taliban's "assaults" on the rights of Afghan women and girls. (Video: Reuters)

What is al-Qaeda’s relationship to the Taliban, and how strong is the group?

The first time the Taliban ruled Afghanistan, from 1996 to 2001, it sheltered al-Qaeda militants who plotted the 9/11 attacks on the United States. The U.S.-led coalition invaded Afghanistan in 2001 with the aim of crushing the extremist group.

After two decades of conflict and counterterrorism operations, “al-Qaeda in Afghanistan is a skeleton of its former self,” said Fawaz Gerges, a professor at the London School of Economics. He said the group lacks charismatic leadership and is “starved financially.”

After Taliban triumph, Biden faces even greater test in preventing extremist resurgence in Afghanistan

A recent U.N. report said al-Qaeda maintained a presence in at least 15 Afghan provinces. An offshoot, al-Qaeda in the Indian Subcontinent, has operated “under the Taliban umbrella” from Kandahar, Helmand and Nimruz provinces, according to the United Nations. In total, al-Qaeda members are estimated to number between several dozen and 500 people.

Mohammad Naeem, a Taliban spokesman, denied the presence of al-Qaeda in Afghanistan in an interview with Saudi Arabia’s al-Hadath TV that aired Sunday. He said the extremist group had no foothold in the country and no relationship with the Taliban, though there may be “family ties” between members of the two organizations.

But a recent U.N. report said al-Qaeda showed “no indication of breaking ties” with the Taliban, with ideological alignment and personal relationships, including intermarriage, keeping the groups close.

Afghanistan analyst Abdul Sayed characterized the relationship as “cordial and stronger than in the pre-9/11 period.”

Though the Taliban has “begun to tighten its control” over al-Qaeda, the United Nations said in the spring, “it is impossible to assess with confidence that the Taliban will live up to its commitment to suppress any future international threat emanating from Al-Qaida in Afghanistan.”

Nearly 20 years of war, 10 days to fall: Afghanistan, by the numbers

What presence does the Islamic State have in Afghanistan?

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

Started by Pakistani national Hafiz Saeed Khan, who pledged allegiance to the Islamic State’s then-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. Shiites are particularly frequent targets.

U.N. Secretary General António Guterres warned in 2019 that after the Islamic State lost its territory in Iraq and Syria, its umbrella group in Afghanistan had access to hundreds of millions of dollars to finance terrorism.

ISIS-K has never successfully captured territory in Afghanistan. Instead, its strategy has centered on attacking civilian targets such as mosques, schools and weddings.

School bombing heightens fears among Afghanistan’s Hazaras, long a target for militants, amid U.S. exit

The number of attacks it has carried out annually has decreased in recent years. During the first four months of this year, the United Nations. recorded 77 attacks associated with the group. The Islamic State claimed responsibility for the bombing of a school for girls in Kabul in May that killed more than 85 people, mostly students.

U.S. airstrikes took out key leaders of the Islamic State 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.

Still, the affiliate group has managed to sustain itself. The United Nations estimates that it retains a core group of some 1,500 to 2,200 fighters in Konar and Nangahar provinces, with smaller cells scattered throughout the country.

Taliban says it will be more tolerant toward women. Some fear otherwise.

What threats do the Islamic State and al-Qaeda pose in Afghanistan?

Most analysts agree that al-Qaeda lacks the strength and capability to pose an immediate threat to the United States.

Secretary of State Antony Blinken said on “Fox News Sunday” that al-Qaeda’s capacity to attack the United States or its allies is “vastly diminished,” though he acknowledged that “remnants” of the group remained in Afghanistan.

But given the chance that al-Qaeda will acquire a sanctuary under the Taliban — and the complications to counterterrorism operations that the Taliban victory poses — some say the group could reconstitute itself.

U.S. intelligence officials had previously said that would take up to two years. But Nathan Sales, who served as a senior counterterrorism official during the Trump administration, said that after the Taliban’s takeover, that period could be around six months.

Much depends on how much leeway the Taliban gives the group.

Sayed said al-Qaeda is “fully following the Taliban’s instructions for supporting its strategies” and supported the Taliban’s February 2020 deal with the United States for the withdrawal of U.S. and allied troops. That agreement obligated the Taliban to prevent al-Qaeda and other terrorists from using Afghanistan to attack the United States or its allies. The Taliban reiterated this week that it remains committed to that promise.

“The Taliban will be unlikely to allow al-Qaeda to operate from Afghanistan and endanger the survival of their nascent rule as the terrorist organization did in 2001,” Gerges said.

Who is Taliban Supreme Leader Haibatullah Akhundzada?

Michael Kugelman, deputy director of the Asia Program at the Wilson Center, said the Islamic State offshoot in Afghanistan is “certainly resilient and potent” but is unlikely to be able to plan attacks on faraway targets.

Afghanistan’s neighbors, however, are concerned about extremist activity. Russia has ramped up military exercises in Tajikistan, which shares a long border with Afghanistan, amid concerns about radical Islamist groups spilling over into its Central Asian ally.

Biden said last week that officials were keeping a close eye on threats from ISIS-K, particularly after the Taliban released many prisoners this month.

In recent days, U.S. intelligence officials anticipated an ISIS-K attack but had limited means to thwart an assault as they rushed to help airlift tens of thousands of people out of Kabul.

Unlike with its attitude toward al-Qaeda, the Taliban sees the Islamic State as an existential threat and has fought the group in Afghanistan for years. Biden on Thursday emphasized that he has seen no evidence to suggest any collusion between the groups.

Kugelman said the Taliban has “compelling reasons” to target the Islamic State and could use newly acquired U.S.-made weapons to do so.

But as it takes up the levers of government, the Taliban may be distracted by more-pressing priorities, experts say, leaving security gaps. The United Nations has estimated that there are already between 8,000 and 10,000 fighters belonging to various militant groups in Afghanistan.

“The U.S. withdrawal is an increasingly galvanizing moment for these jihadist forces in Afghanistan and the broader region,” Kugelman said.

A desire to drive the United States out of the region has long been a focus of propaganda for radical Islamist groups, and Kugelman said the U.S. pullout may inspire militants from the surrounding region to plan local attacks or move to Afghanistan.

There has already been excited chatter among sympathizers of extremist groups in the area. An intelligence official from an Arab nation, speaking on the condition of anonymity to describe evolving assessments, told The Washington Post that officials had seen an uptick in Islamist militant communications about developments in Afghanistan and that the Taliban takeover “is encouraging many jihadists to think about traveling to Afghanistan now instead of Syria or Iraq.”

One al-Qaeda fighter, who goes by the name Abu Khaled, hailed the Taliban’s victory as a turning point for extremist groups.

“God willing, the success of the Taliban will be also a chance to unify mujahideen movements like al-Qaeda and Daesh,” he said, using another name for the Islamic State.

Missy Ryan, Souad Mekhennet, John Hudson and Ellen Francis contributed to this report, which has been updated.