Joe Biden bowed his head at the lectern, as if the weight of the world were on his shoulders. In his breast pocket was a card that keeps a tally of American service members who have died in Afghanistan and Iraq. Now, he knew, it would include people who died serving under him as commander-in-chief.

Addressing the nation from the East Room of the White House, Biden had just endured the bleakest day of his young presidency. In Kabul a suicide bomber and gunmen killed 13 US troops, more than a hundred Afghan civilians, two British nationals and the child of a British national.

Like many presidents before him, Biden was required to move rapidly through emotional keys. There was compassion for the US war dead – “Heroes who have been engaged in a dangerous, selfless mission to save the lives of others” – then a dark warning for terrorists reminiscent of George W Bush: “We will not forgive. We will not forget. We will hunt you down and make you pay.”

A day later Biden made good on his promise of violent retribution when he ordered a drone strike on an Islamic State planner in Afghanistan’s Nangarhar province.

After half a century in national politics, Biden found himself tested as never before. He came into office in January identifying multiple crises but did not mention Afghanistan in his inaugural address. The unforeseen catastrophe has diminished his standing at home and America’s stature abroad.

It was just last month that the president, implacably resolved to withdraw US forces by 31 August, declared that “the Taliban overrunning everything and owning the whole country is highly unlikely”, only to watch impotently as the extremists did just that, upending two decades of human rights gains in only two weeks.

America’s allies, who assumed that the defeat of Donald Trump would at least Make America Competent Again, felt blindsided. The US struck an uneasy truce with the Taliban and undertook what may prove the biggest airlift in history, evacuating more than 100,000 people from the crowded, chaotic Kabul airport.

Then, on Thursday, the Islamic State group struck, inflicting the deadliest attack on US forces in Afghanistan in a decade and killing dozens of Afghans desperate to flee.

As horrifying images flashed around the world, the White House sought to project calm. News of the attacks broke as Biden’s national security team were in the situation room. When the president walked in, one of the first updates he received was about the atrocity in Kabul. Details came in through the day, from commanders on the ground.

Jen Psaki, the White House press secretary, told reporters on Thursday evening: “Any day where you lose service members may be the worst day of your presidency, and hopefully there’s not more. But we are certainly early in the presidency at this point in time. So I would say he was sombre and, as he said today, outraged at these terrorists taking the lives of service members.”

It was a day of raw emotion unlike any other in the Biden White House. The Politico website reported: “During one staff meeting, sniffles could be heard as various staffers fought back tears when they learned of the [American] deaths, according to a person close to the situation. One White House official described the pace of the day’s events as overwhelming.”

The 78-year-old president, whose own life is freighted with personal tragedies, would have been hit hard by the losses. Leon Panetta, a former CIA director and defense secretary under Barack Obama, can recall standing alongside then Vice-President Biden when America’s war dead were flown home to Dover air force base in Delaware.

Panetta said on Friday: “It was clear to me that he really felt deeply about the impact the loss of a loved one had on these families. I’m sure it’s something that he’s going to have to carry now for these next number of days as president, recognising that the loss of life took place during his presidency.”

He added: “It had to be President Biden’s worst nightmare to have lost that many US service people in one blow when his hope, I think, had always been to be able to bring our presence in Afghanistan to an end without losing another American. To have that happen I’m sure probably represented his worst day in office.”

For now the frantic evacuation continues, under constant threat of further attacks, racing to meet the deadline of 31 August. There is little doubt some US citizens and many thousands of Afghan allies will still be in the country after that date. Their fate remains deeply uncertain and could cast a lasting pall over Biden’s legacy.

The debacle has drawn the harshest rebukes of his seven-month presidency, with some Republicans demanding his impeachment or resignation. Trump called the retreat from Afghanistan “the single greatest humiliation in our history”. Congresswoman Elise Stefanik insisted that Biden had “blood on his hands”.

Democratic leaders have generally remained diplomatic but some members of Congress have spoken out or at least urged an extension to the deadline.

Seth Moulton, a US Marine Corps veteran who made a surprise trip to Kabul airport this week, told New York magazine: “The thing that everybody needs to understand, even if you completely agree with the Biden administration’s decision to withdraw, the way they have handled this has been a total fucking disaster. It will be measured in bodies, because a lot of people are dying because they can’t get out.”

Other western nations rapidly wound down evacuation efforts after the bombing. For allies there is bitter disappointment: Biden had been seen as a steady hand who could restore US leadership on the world stage. Scathing comments expressed at a recent session of the British parliament made clear how faith in American hegemony has been shaken.

Panetta, co-founder of the Panetta Institute for Public Policy, said: “I don’t think there’s any question that when the pictures of that chaos at the airport were shown around the world it impacted on the credibility of the United States, particularly the sense that our allies, frankly, did not play much of a role in terms of the decision itself and obviously were not part of a lot of the strategising that took place. I think he’s got some work cut out for him to try restore credibility, particularly with our allies.”

Yet to the dismay of many Biden has dug in, given no inch, acknowledged no error and shown no contrition. He insists a deal struck by Trump with the Taliban, for US forces to leave by May, left him with only two choices: withdraw or escalate. The American public, exhausted by America’s longest war, had no desire for the latter.

Some observers have been puzzled that Biden’s much-vaunted empathy, displayed for American troops, has been less copious for Afghan women and girls now at the mercy of the Taliban, and that a man steeped in foreign policy has unapologetically prioritised American interests.

Evan Osnos, a Biden biographer, said: “The moment reflected a collision of two of his instincts: The total, frustrated determination to act on 12 years of wanting out of Afghanistan, combined with a small-c conservative calculation about America’s interests. The fact is, and he would acknowledge this, that he thinks more about members of the American military than he does about Afghans who worked with us or for us. His compassion has limits.”

Biden’s popularity has taken a hit, dipping below 50% for the first time in opinion polls. He is fortunate, however, that the midterm elections that will decide the balance of power in Congress are 15 months away – an eternity in political news cycles. It is far from certain that he will suffer long-term damage for ending a war that had become profoundly unpopular.

Indeed, some Democratic strategists caution against a rush to judgment. Bob Shrum, director of the USC Dornsife Center for the Political Future, said: “There was obviously an intelligence failure, which I don’t think can be blamed on Biden. They’ve done as well as you possibly could under the most adverse circumstances imaginable.

“I mean, this will have been the most successful airlift of evacuees in the history of warfare or maybe in history, period. All these people who are prematurely saying, ‘Oh, well, that’s the end of Biden,’ that’s totally ahistorical and fails to recognise that we’ve been through things like this before.”