In his first congressional testimony on the tumultuous US withdrawal from Afghanistan, Defense Secretary Lloyd Austin on Tuesday defended the military’s execution of a frantic airlift from Kabul and asserted it will be “difficult but absolutely possible” to contain future threats from Afghanistan without troops on the ground.

Speaking alongside Austin, Gen. Mark Milley, chairman of the Joint Chiefs of Staff, cited “a very real possibility” that al-Qaida or the Islamic State group’s Afghanistan affiliate could reconstitute in Afghanistan under Taliban rule and present a terrorist threat to the United States in the next 12 to 36 months.

It was al-Qaeda’s use of Afghanistan as a base from which to plan and execute its attacks on the United States on Sept. 11, 2001, that triggered the US invasion of Afghanistan a month later.

“And we must remember that the Taliban was and remains a terrorist organization and they still have not broken ties with al-Qaida,” Milley said. “I have no illusions who we are dealing with. It remains to be seen whether or not the Taliban can consolidate power or if the country will further fracture into civil war.”

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Austin questioned decisions made over the 20-year course of the U.S. war in Afghanistan. In retrospect, he said, the American government may have put too much faith in its ability to build a viable Afghan government.

“We helped build a state, but we could not forge a nation,” he told the Senate Armed Services Committee. “The fact that the Afghan army we and our partners trained simply melted away – in many cases without firing a shot – took us all by surprise. It would be dishonest to claim otherwise.”

Austin acknowledged shortcomings in the final airlift from Hamid Karzai International Airport that began Aug. 14, such as an initial wave of violence at and near the airfield that led to multiple deaths of Afghan civilians. But he asserted that the airlift was a historic accomplishment that removed 124,000 people from Taliban rule.

“To be clear, those first two days were difficult,” said Austin, who is a veteran of the war. “We all watched with alarm the images of Afghans rushing the runway and our aircraft. We all remember the scenes of confusion outside the airport. But within 48 hours, our troops restored order, and the process began to take hold.”

The Biden administration faces criticism on multiple fronts for its handling of the final months of the war.

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Sen. James Inhofe, the ranking Republican on the Armed Services panel, told Austin and Milley that the withdrawal and evacuation amounted to an “avoidable disaster.”

Republicans in particular have intensified their attacks on President Joe Biden’s decision to pull all troops out of Afghanistan by Aug. 30, saying it left the US more vulnerable to terrorism. They are demanding more details on the suicide bombing in Kabul that killed 13 American service members in the final days of the withdrawal.

Gen. Frank McKenzie, who as head of Central Command oversaw the withdrawal, testified alongside Austin and Milley.

Inhofe has peppered the Pentagon with a lengthy list of questions about multiple aspects of the withdrawal, including the suicide bombing on Aug. 26 at Kabul’s international airport that killed some 169 Afghans in addition to the American service members. He also is demanding information about decision making over the summer as it became apparent that the Taliban were overwhelming US-backed Afghan forces.

“We need a full accounting of every factor and decision that led us to where we are today and a real plan for defending America moving forward,” Inhofe wrote last week.

The withdrawal ended the longest war in US history. The Biden administration, and some Democrats in Congress, have argued that former President Donald Trump bears some of the blame for the war ending in a Taliban victory, since his administration signed a deal with the Taliban in 2020 that promised a full American withdrawal by May 2021.

They also have pointed to a yearslong US failure to build an Afghan military that could stand up to the Taliban.