Defense Secretary Lloyd Austin, in his opening statement before the Senate Armed Services Committee's hearing on Afghanistan, defended the timeline and tactics of the U.S. military's withdrawal ahead of Aug. 31 but acknowledged the Taliban takeover the country happened much faster than leaders had expected.

"We helped build a state, Mr. Chairman, but we could not forge a nation," he said. "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. And it would be dishonest to claim otherwise."

He then listed what he called "uncomfortable truths."

"That we did not fully comprehend the depth of corruption and poor leadership in their senior ranks, that we did not grasp the damaging effect of frequent and unexplained rotations by President Ghani of his commanders, that we did not anticipate the snowball effect caused by the deals that Taliban commanders struck with local leaders in the wake of the Doha agreement, and that the Doha agreement itself had a demoralizing effect on Afghan soldiers, and finally, that we failed to fully grasp that there was only so much for which and for whom many of the Afghan forces would fight," he said.

Austin said staying past Aug. 31 would have "would have greatly imperiled our people and our mission" as the Taliban's offer to cooperate ended on Sept. 1, opening U.S. troops to Taliban attack.

He also took on the issue of why Bagram Air Base was not kept open, saying it would have taken 5,000 troops to secure it and, being 30 miles from Kabul, would have significantly helped the main mission of protecting the embassy.

Gen. Mark Milley, chairman of the Joint Chiefs of Staff, will face tough questions Tuesday from the Senate Armed Services Committee on Tuesday on the U.S. military’s chaotic exit from Afghanistan. He’s also expected to address reporting that he went outside the regular chain of command with calls to China in the final days of former President Donald Trump’s presidency.

Defense Secretary Lloyd Austin and Gen. Kenneth McKenzie, commander of U.S. Central Command, are also scheduled to appear before the Senate panel alongside Milley. Senators are expected to press the top Pentagon leaders on decisions surrounding the evacuation and of ongoing threats of terrorism in Afghanistan without a U.S. presence on the ground.

It's been nearly one month since President Joe Biden withdrew all U.S. troops, ending an evacuation operation after the Afghan government collapsed and the Taliban seized the capital Kabul. In those final days, a U.S. drone strike killed at least 10 Afghan civilians, including seven children, an event lawmakers are expected to press military leaders upon on Tuesday.

At least 236 passengers were on an evacuation flight from Kabul to Qatar this weekend, which officials said is the largest passenger evacuation since the Aug. 31 U.S. withdrawal deadline.

Evacuees included U.S., U.K., German, Irish, French nationals and more, according to a senior Qatari government official.

"The State of Qatar is pleased to announce that following extensive consultation with parties on the ground and international partners, just moments ago the fourth passenger flight departed Kabul International Airport to Doha’s Hamad International Airport (HIA)," the official said in a statement Sunday.

"The State of Qatar will continue its collaboration with international partners on efforts that ensure freedom of movement in Afghanistan, while working with various parties on the ground towards more general progress in the country moving forward," the official said.

-ABC News' Sohel Uddin

A drone strike near the Kabul airport last month that killed 10 Afghans was "a mistake," U.S. Central Command Gen. Frank McKenzie admitted at a briefing Friday.

The U.S. believed it was targeting a terrorist in the Aug. 29 strike, but McKenzie said it is unlikely that those who died or the targeted vehicle were associated with ISIS-K "or were a direct threat to U.S. forces."  

As many as 10 civilians, including up to seven children, died in the strike, according to McKenzie.

"I offer my profound condolences to the family and friends of those who were killed," the general said.