The top U.S. military officer called the 20-year war in Afghanistan a "strategic failure" and acknowledged to Congress that he had favored keeping several thousand troops in the country to prevent a collapse of the U.S.-supported Kabul government and a rapid takeover by the Taliban.

Republicans on the Senate Armed Services Committee pointed to the testimony Tuesday by Gen. Mark Milley, chairperson of the Joint Chiefs of Staff, as evidence that President Joe Biden had been untruthful when, in a television interview last month, he suggested the military had not urged him to keep troops in Afghanistan.

Milley refused to say what advice he gave Biden last spring when Biden was considering whether to comply with an agreement the Trump administration had made with the Taliban to reduce the American troop presence to zero by May 2021, ending a U.S. war that began in October 2001.

Testifying alongside Milley, Defense Secretary Lloyd Austin also refused to reveal his advice to Biden. Milley told the committee, when pressed, that it had been his personal opinion that at least 2,500 U.S. troops were needed to guard against a collapse of the Kabul government and a return to Taliban rule.

Defying U.S. intelligence assessments, the Afghan government and its U.S.-trained army collapsed in mid-August, allowing the Taliban, which had ruled the country from 1996 to 2001, to capture Kabul with what Milley described as a couple of hundred men on motorcycles, without a shot being fired.

That triggered a frantic U.S. effort to evacuate American civilians, Afghan allies and others from Kabul Hamid Karzai International Airport. Gen. Frank McKenzie, who as head of Central Command was overseeing U.S. troops in Afghanistan, said he shared Milley’s view that keeping a residual force there could have kept the Kabul government intact.

"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 six-hour Senate hearing marked the start of what is likely to be an extended congressional review of the U.S. failures in Afghanistan. The length and depth of the hearing stood in contrast to years of limited congressional oversight of the war and the hundreds of billions of taxpayer dollars it consumed.

"The Republicans' sudden interest in Afghanistan is plain old politics," said Sen. Elizabeth Warren, a Massachusetts Democrat, who supported Biden's decision to end U.S. involvement there.

Austin and Milley are scheduled to appear Wednesday before the House Armed Services Committee to review the war. The hearing at times was 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 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.

When asked in that interview whether military advisers had recommended keeping 2,500 troops in Afghanistan, Biden replied, "No. No one said that to me that I can recall." He also said the advice "was split."

White House press secretary Jen Psaki said Tuesday that Biden was referring to having received a range of advice. "Regardless of the advice, it’s his decision, he’s the commander in chief," she said. 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. Republican Sen. Tom Cotton of Arkansas asked Milley why he did not choose to resign after his advice was rejected.

Milley, who was appointed to his position as chairperson 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."

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. Milley cited "a very real possibility" that al-Qaida or the Daesh'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.