U.S. Defense Secretary Lloyd Austin told Congress on Tuesday that the Afghan army's sudden collapse caught the Pentagon "by surprise," as military leaders confronted a contentious Senate hearing about how and why America lost its longest war.

Republican lawmakers accused President Joe Biden of lying about recommendations from his military that some troops should be kept in Afghanistan. Even Biden's Democrats expressed frustration with a chaotic withdrawal that left U.S. troops dead and American citizens behind.

General Mark Milley, chairman of the Joint Chiefs of Staff, and General Frank McKenzie of U.S. Central Command also acknowledged being caught off-guard by the speed of the Taliban takeover and collapse of the U.S.-backed government in Kabul.

It was their first public congressional testimony since the Islamic Emirate of Afghanistan (IEA) won the war in August.

"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," Austin, told the Senate Armed Services Committee.

"It would be dishonest to claim otherwise."

McKenzie and Milley testified that they had believed it would have been best to keep a minimum of 2,500 troops in the country. In an August interview Biden denied his commanders had recommended that, saying: "No. No one said that to me that I can recall."

Milley refused to be drawn on whether Biden had lied when pressed by Republican Senator Dan Sullivan

"I'm not going to categorize a statement of the President of the United States," Milley said.

He said that the U.S. missed warning signs about the coming failure of "leadership and will" in its Afghan allies that ultimately led to their collapse.

Republican Senator Joni Ernst said Biden's decision to keep former President Donald Trump's unconditional withdrawal agreement with the IEA had squandered U.S. sacrifices for what he thought would be "a cheap political victory."

"The loss of our service members, and abandonment of Americans and Afghan allies last month was an unforced, disgraceful humiliation that didn't have to happen," Ernst said.

Senator James Inhofe, the panel's top Republican, described it as a "horror of the president's own making."

White House spokesperson Jen Psaki said Biden's military experts had provided "a range of viewpoints" about Afghanistan, and that Biden believed leaving troops there would "mean war with the Taliban (IEA)."

Milley, the top U.S. military officer, noted military warnings since late 2020 that an accelerated, unconditional withdrawal could precipitate the collapse of the Afghan military and government.

"That was a year ago. My assessment remained consistent throughout," Milley said.

Austin, Milley and senators - many of whom oversaw the war effort for years - seemed full of questions about what went wrong, citing failures to appreciate the impact of corruption and damaged morale in the ranks.

"There's a series of strategic lessons to be learned," Milley said.

Democrats faulted Republicans for blaming Biden, who has been president since January, for everything that went wrong during the 20 years U.S. troops have been in Afghanistan, including under Trump.

"Anyone who says the last few months were a failure, but everything before that was great, clearly hasn't been paying attention," Democratic Senator Elizabeth Warren said.

Austin praised U.S. personnel who helped airlift 124,000 people out of the country.

But Milley acknowledged that while the evacuation effort was a logistical accomplishment, the withdrawal was a "strategic defeat".

He warned that the IEA has not broken ties with al Qaeda.

A reconstituted al Qaeda in Afghanistan with aspirations to attack the United States was "a very real possibility" - perhaps in as little as a year, he said.