The rapid fall of Afghanistan to Taleban control presents Britain and its Nato allies with two primary dilemmas — the first is immediately pressing, the second has longer-term implications.

The urgent task is to determine a policy for Afghans seeking refuge. Although there are many calls, from lawmakers and the media, to offer more support, asylum policy has been fraught political ground in both the UK and Europe for years. It doesn't take much for the magnanimity that surfaces in the early days of a humanitarian crisis to turn into ennui, indifference and even resentment.

During the 2015 Syrian refugee crisis, Germany’s Angela Merkel was praised abroad but heavily criticised at home for her open-door policy, in which more than a million refugees — mainly from Syria, Iraq and Afghanistan — were accepted into Germany. That precipitated not only a decline in popularity for her party but also a crisis over borders within the European Union that is still felt today. The refugee crisis also formed the basis of a major pillar of the 2016 Brexit campaign.

Neither Europe nor Britain want a repeat. Greece, which was on the front lines of accepting refugees in 2015, has made clear it doesn't want to become a gateway for Afghans fleeing the Taleban. France is also among the wary. On Monday, President Emmanuel Macron, who is fighting a close-run presidential election campaign, spoke of needing to protect against “significant irregular migratory flows”.

Merkel, soon to retire, has been more magnanimous, reportedly supporting the evacuation of up to 10,000 Afghans. But her party's general secretary, Paul Ziemiak, was more circumspect.

Boris Johnson faces a big test, too. The next parliamentary session will see a heated debate about Britain’s role, from its own military withdrawal to its plan to help Afghans fleeing the Taleban. Britain’s Afghanistan ambassador, Laurie Bristow, is still in Kabul processing visas. There is cross-party support for a generous refugee policy, but the devil is always in the detail.

Foreign Secretary Dominic Raab described Britain as a “big-hearted nation”, but Britain's longtime policy of denying settlement to Afghans and even forcibly returning them doesn’t exactly speak to that. There is already criticism Britain did too little to arrange for interpreters to leave the country after dozens of former military commanders urged their resettlement applications be accepted. Johnson even had to personally intervene this weekend, after a social media uproar, to ensure that a small group of exceptional female scholarship winners could receive their visas as promised.

The Government is set to announce a settlement scheme modelled on the 2015 policy to help 20,000 Syrians resettle in Britain over five years. But there are concerns that’s too long a time frame. The Syrian refugee programme also required the assessment capacities of the United Nations, and it is not clear how it would work this time round. There is a danger, as Britain’s former national security adviser Mark Sedwill has warned, that Afghans would be stuck between the Taleban or an eternity in refugee camps. The goal over the next year is reportedly to resettle a modest 5,000 refugees.

And then there is longer-term challenge of how Britain and Europe adjust to a security relationship with the US that has significantly changed. Donald Trump famously questioned the purpose of their alliance, but Biden’s election seemed to signal a return to collaboration. It hasn’t, really.

Although one can debate the cost and record of such interventions, the US’s willingness to lead a coalition into places like Afghanistan or Iraq sent a message to the world’s rogue regimes that there was a guard of sorts on duty and that America defined its interests broadly.

As Latvian defence minister Artis Pabriks summed it up, “This era is over.” The Afghan withdrawal — the decision but also the manner in which it was carried out and defended by Biden — represents a sharp diminution in American influence. It signals a future without efforts at long-term peacekeeping or to uphold a rules-based international order.

This is the reality that has to be factored into Britain’s foreign policy and that of its other Nato allies. You only had to hear Hamas’s congratulatory messages to the Taleban to see how the fall of Kabul will embolden jihadists and extremists around the world. If Afghanistan proves ungovernable and the surrounding region more contested — both highly likely — global terrorism could become a resurgent threat.

Ironically, Britain just completed a multiyear foreign and security strategy review in March. The 114-page report mentioned Afghanistan only briefly: “We will continue to support stability in Afghanistan as part of a wider coalition.”

That proved naive. As Defence Secretary, Ben Wallace has made clear, Britain would have preferred to keep an Afghan mission going. Turkey and Italy were also reportedly keen, but America’s Nato allies were divided. And there was little that a smaller coalition could do without US air support. Yet that raises the question: what, apart from an outright attack on a major European power, would Allies actually mobilise for?

It would be wrong to say these challenges don’t also present opportunities. Britain, which holds the G7 presidency at present, should lead a robust international humanitarian effort to help resettle the most vulnerable Afghans. Former Tory lawmaker and Afghan expert Rory Stewart places the number of potential refugees in the millions.

At the same time, Britain must adjust to the limitations of its own much reduced military capabilities and America’s reduced appetite for intervention. That requires a greater emphasis on co-operation and burden-sharing with Europe, which may help both sides move past the Brexit squabbles that have hurt trust.

It will take a long time for the US to repair the damage the Afghan withdrawal has done to both the region and its Western alliance. In the meantime, Britain and its European allies will need to reckon with the consequences.

• Therese Raphael is a columnist for Bloomberg Opinion