The top U.S. military officer called the 20-year war in Afghanistan a “strategic failure” and told Congress on Tuesday that he believes the U.S. should have kept several thousand troops in the country to prevent the unexpectedly rapid takeover by the Taliban.

In his first congressional testimony on the tumultuous withdrawal, Gen. Mark Milley, chairman of the Joint Chiefs of Staff, refused to say what advice he gave President Joe Biden last spring when Biden was considering whether to keep any troops in Afghanistan.

But he told the Senate Armed Services Committee it was his personal opinion that at least 2,500 were needed to guard against a collapse of the Kabul government and a return to Taliban rule.

In a blunt assessment of a war that cost 2,461 American lives, Milley said the result was years in the making.

“Outcomes in a war like this, an outcome that is a strategic failure — the enemy is in charge in Kabul, there's no way else to describe that — that is a cumulative effect of 20 years,” he said, adding that lessons need to be learned, including whether the U.S. military made the Afghans overly dependent on American technology in a mistaken effort to make the Afghan army look like the American army.

Gen. Frank McKenzie, who as head of Central Command had overseen the final months of the U.S. war, said he agreed with Milley that a few thousand troops should have been kept in Afghanistan, despite the Trump administration's agreement with the Taliban in 2020 that all U.S. troops would leave by May 2021.

"I recommended that we maintain 2,500 troops in Afghanistan, and I also recommended early in the fall of 2020 that we maintain 4,500 at that time, those were my personal views,” McKenzie said. “I also had a view that the withdrawal of those forces would lead inevitably to the collapse of the Afghan military forces and eventually the Afghan government.”

The Senate hearing was at times contentious, as Republicans sought to portray Biden as having ignored advice from military officers and mischaracterized the military options he was presented last spring and summer. Several Republicans tried unsuccessfully to draw Milley, McKenzie and Defense Secretary Lloyd Austin into commenting on the truthfulness of Biden's statement to ABC News on Aug. 18, three days after the Taliban took control of Kabul, that no senior military commander had recommended against a full troop withdrawal when it was under discussion in the first months of Biden's term.

Sen. Tom Cotton, R-Ark., asked Milley why he did not choose to resign after his advice was rejected.

Milley, who was appointed to his position as chairman of the Joint Chiefs of Staff by President Donald Trump and retained by Biden, said it was his responsibility to provide the commander in chief with his best advice.

“The president doesn't have to agree with that advice,” Milley said. “He doesn't have to make those decisions just because we are generals. And it would be an incredible act of political defiance for a commissioned officer to resign just because my advice was not taken."

Testifying alongside Milley and McKenzie, Austin defended the military's execution of a frantic airlift from Kabul in August and asserted it will be “difficult but absolutely possible” to contain future threats from Afghanistan without troops on the ground. Under questioning, he, too, declined to say what advice he had given Biden about whether to make a full troop withdrawal.

Milley 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-Qaida'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 U.S. 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.”

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 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.”

Asked why the United States did not foresee the rapid collapse of the Afghan army, Milley said that in his judgment the U.S. military lost its ability to see and understand the true condition of the Afghan forces when it ended the practice some years ago of having advisers alongside the Afghans on the battlefield.

“You can’t measure the human heart with a machine, you have to be there,” Milley said.

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 process began to take hold.”

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

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 Biden’s decision to pull all troops out of Afghanistan by Aug. 30, saying it left the U.S. 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.