WASHINGTON (NYTIMES) - United States President Joe Biden's top advisers concede they were stunned by the rapid collapse of the Afghan army in the face of an aggressive, well-planned offensive by the Taleban that now threatens Kabul, Afghanistan's capital.

The past 20 years show they should not have been.

If there is a consistent theme over the two decades of war in Afghanistan, it is the overestimation of the results of the US$83 billion (S$112.59 billion) that the United States has spent since 2001 on training and equipping the Afghan security forces, and an underestimation of the brutal, wily strategy of the Taleban.

The Pentagon issued dire warnings to Mr Biden, even before he took office, about the potential for the Taleban to overrun the Afghan army, but intelligence estimates - now shown to have badly missed the mark - assessed that it might happen in 18 months, not weeks.

Commanders knew that the afflictions of the Afghan forces had never been cured: the deep corruption, the failure by the government to pay many Afghan soldiers and police officers for months, the defections, and the soldiers sent to the front without adequate food and water, let alone arms.

In the past several days, the Afghan forces steadily collapsed as they battled to defend ever-shrinking territory, losing Mazar-e-Sharif, the country's economic engine, to the Taleban on Saturday (Aug 14).

Mr Biden's aides say the persistence of those problems reinforced his belief that the US could not prop up the Afghan government and military in perpetuity. In Oval Office meetings this spring, he told aides that staying another year, or even five, would not make a substantial difference and was not worth the risks.

In the end, an Afghan force that did not believe in itself and a US effort that Mr Biden and most Americans no longer believed would alter the course of events combined to bring an ignoble close to America's longest war.

The US kept forces in Afghanistan far longer than the British did in the 19th century, and twice as long as the Soviets - with roughly the same results.

For Mr Biden, the last of four US presidents to face painful choices in Afghanistan, but the first to get out, the debate about a final withdrawal and the miscalculations over how to execute it began the moment he took office.

Under former president Donald Trump, "we were one tweet away from complete, precipitous withdrawal", said Mr Douglas Lute, a retired general who directed Afghan strategy at the National Security Council for presidents George W. Bush and Barack Obama.

"Under Biden, it was clear to everyone who knew him, who saw him pressing for a vastly reduced force more than a decade ago, that he was determined to end US military involvement, but the Pentagon believed its own narrative that we would stay forever.

"The puzzle for me is the absence of contingency planning: If everyone knew we were headed for the exits, why did we not have a plan over the past two years for making this work?"

A sceptical President

From the moment that news outlets called Pennsylvania for Mr Biden on Nov 7, making him the next commander-in-chief for 1.4 million active-duty troops, Pentagon officials knew they would face an uphill battle to stop a withdrawal of US forces from Afghanistan.

Defence Department leaders had already been fending off Mr Trump, who wanted a rapid drawdown. In a Twitter post last year, he declared all US troops would be out by that Christmas.

And while they had publicly voiced support for the agreement that Mr Trump reached with the Taleban in February 2020 for a complete withdrawal this May, Pentagon officials said they wanted to talk Mr Biden out of it.

After Mr Biden took office, top Defence Department officials began a lobbying campaign to keep a small counter-terrorism force in Afghanistan for a few more years.

They told the President that the Taleban had grown stronger under Mr Trump than at any point in the past two decades, and pointed to intelligence estimates predicting that in two or three years, al-Qaeda could find a new foothold in Afghanistan.

Shortly after Mr Lloyd Austin was sworn in as defence secretary on Jan 22, he and General Mark Milley, chairman of the Joint Chiefs of Staff, recommended to Mr Biden that 3,000 to 4,500 troops stay in Afghanistan, nearly double the 2,500 troops there.

On Feb 3, a congressionally appointed panel led by retired four-star Marine general Joseph Dunford publicly recommended that Mr Biden abandon the exit deadline of May 1 and further reduce US forces only as security conditions improved.

The president told his national security team, including Secretary of State Antony Blinken and National Security Adviser Jake Sullivan, that he was convinced that no matter what the US did, Afghanistan was almost certainly headed into another civil war - one that Washington not only could not prevent, but also, in his view, could not be drawn into.

By March, Pentagon officials said they realised they were not getting anywhere with Mr Biden. Although he listened to their arguments and asked extensive questions, they said they had a sense that his mind was made up.

In late March, Mr Austin and Gen Milley made a last-ditch effort with the President, by forecasting dire outcomes in which the Afghan military folded in an aggressive advance by the Taleban. They drew comparisons to how the Iraqi military was overrun by the Islamic State in Iraq and Syria terror group in 2014, after US combat troops left Iraq, prompting Mr Obama to send US forces back.

"We've seen this movie before," Mr Austin told Mr Biden, according to officials with knowledge of the meetings.

But the President was unmoved. If the Afghan government could not hold off the Taleban now, aides said he asked, when would they be able to? None of the Pentagon officials could answer the question.

On the morning of April 6, Mr Biden told Mr Austin and Gen Milley he wanted all US troops out by Sept 11.

The intelligence assessments in Mr Biden's briefing books gave him some assurance that if a bloody debacle resulted in Afghanistan, it would at least be delayed. As recently as late June, the intelligence agencies estimated that even if the Taleban continued to gain power, it would be at least a year and a half before Kabul would be threatened; the Afghan forces had the advantages of greater numbers and air power, if they could keep their helicopters and planes flying.

Even so, the Pentagon moved swiftly to get its troops out, fearful of the risks of leaving a dwindling number of Americans in Afghanistan and of service members dying in a war the US had given up for lost.

Before the July 4 weekend, the US had handed over Bagram Air Base, the military hub of the war, to the Afghans, effectively ending all major US military operations in the country.

"Afghans are going to have to be able to do it themselves with the air force they have, which we're helping them maintain," Mr Biden said at the time. A week later, he argued that the Afghans "have the capacity" to defend themselves.

"The question is," he said, "will they do it?"

The will is gone

To critics of the decision, the President underestimated the importance of even a modest presence, and the execution of the withdrawal made the problem far worse.

"We set them up for failure," said Mr David Petraeus, a retired general who commanded the international forces in Afghanistan from 2010 until he was appointed director of the Central Intelligence Agency the next year.

Mr Biden's team, he argued, "did not recognise the risk incurred by the swift withdrawal" of intelligence and reconnaissance drones and close air support, as well as the withdrawal of thousands of contractors who kept the Afghan air force flying - all in the middle of a particularly intense fighting season.

The result was that Afghan forces on the ground would "fight for a few days, and then realise there are no reinforcements" on the way, he said. The "psychological impact was devastating", he added.

But administration officials, responding to such critiques, counter that the Afghan military dwarfs the Taleban, some 300,000 troops to 75,000.

They have an air force, a capable air force, something the Taleban does not have, Pentagon Press Secretary John Kirby said last Friday. "They have modern equipment. They have the benefit of the training that we have provided for the last 20 years. It's time now to use those advantages."

But by the time Mr Kirby noted those advantages, none of them seemed to be making a difference. Feeling abandoned by the US and commanded by rudderless leaders meant that Afghan troops on the ground "looked at what was in front of them, and what was behind them, and decided it's easier to go off on their own", said retired general Joseph Votel, a former commander of US Central Command who oversaw the war in Afghanistan from 2016 to 2019.

Mr Biden, an administration official said, expressed frustration that President Ashraf Ghani of Afghanistan had not managed to effectively plan and execute what was supposed to be the latest strategy: consolidating forces to protect key cities.

On Wednesday, Mr Ghani fired his army chief Wali Mohammad Ahmadzai, who had been in place for only two months, replacing him with Major-General Haibatullah Alizai, a special operations commander. The commandos under Maj-Gen Alizai are the only troops who have consistently fought the Taleban these past weeks.

Mr Richard Fontaine, chief executive of the Centre for a New American Security, an influential Washington think-tank that specialises in national security, wrote that in the end, the 20-year symbiosis between the US and the Afghan government it stood up for, supported and ushered through elections had broken down.

"Those highlighting the Afghan government's military superiority - in numbers, training, equipment, air power - miss the larger point," he wrote recently. "Everything depends on the will to fight for the government. And that, it turns out, depended on US presence and support. We're exhorting the Afghans to show political will when theirs depends on ours. And ours is gone."

On Saturday, as the last major city in northern Afghanistan fell to the Taleban, Mr Biden accelerated the deployment of 1,000 more troops to the country to help ensure the safe evacuation of US citizens and Afghans who worked for the US government from Kabul.

Mr Biden released a lengthy statement in which he blamed Mr Trump for at least part of the unfolding disaster. He said: "I inherited a deal cut by my predecessor (which) left the Taleban in the strongest position militarily since 2001 and imposed a May 1, 2021, deadline on US forces."

He said when he took office, he had a choice: abide by the deal or "ramp up our presence and send more American troops to fight once again in another country's civil conflict".

He said: "I was the fourth president to preside over an American troop presence in Afghanistan - two Republicans, two Democrats. I would not, and will not, pass this war onto a fifth."