LONDON (AP) — The messy exit of Western military forces from Afghanistan and the swift takeover of the country by the Taliban has stunned officials in Britain and strained the U.K.'s “special relationship” with its most important ally, the United States.

London’s powerlessness, so far, to change Washington’s course also is a blow to U.K. hopes that an assertive “Global Britain” will be a major global player in the wake of its exit from the European Union.

The main stumbling block ahead of an emergency summit of Group of Seven leaders on Tuesday is a deadline for the U.S. effort to evacuate thousands of Americans, Afghans and others from Kabul. U.S. troops are scheduled to end their mission on Aug. 31, and Britain wants an extension. President Joe Biden left open that possibility, but the Taliban call the date a “red line,” saying that prolonging the American presence would “provoke a reaction.”

Prime Minister Boris Johnson called the virtual G-7 meeting in hope of having some impact on the chaotic course of events. Britain currently holds the presidency of the club of wealthy nations.

Johnson and Biden spoke by telephone Monday, and Johnson's office said the leaders “agreed to continue working together to ensure those who are eligible to leave are able to, including after the initial phase of the evacuation has ended.” It did not elaborate.

British officials acknowledge the limits of their influence, saying the airlift will end when American troops depart.

“We have previously spoken to other NATO countries about staying, and that didn’t prove to be a viable option,” said Johnson’s spokesman, Max Blain. “We are going to need to leave the airport at the same time as the Americans.”

Defense Secretary Ben Wallace, who has called the U.S. deal with the Taliban that set the Aug. 31 deadline a “mistake,” struck an almost pleading tone, saying that if Biden extended the operation “even by a day or two, that will give us a day or two more to evacuate people.”

“Because we are really down to hours now, not weeks, and we have to make sure we exploit every minute to get people out.”

About 1,000 British soldiers are stationed at Kabul’s airport, alongside about 6,000 American troops, to manage the evacuation.

Senior U.K. military officers have expressed anger over the U.S. pullout from Afghanistan, saying it exposes the hollowness of the trans-Atlantic “special relationship” — a phrase used since World War II to stress the bonds of history, friendship and shared diplomatic interests between London and Washington.

More than 150,000 British troops served in Afghanistan in the years after the U.S.-led 2001 invasion — the largest contingent after the Americans — and 457 died in the campaign.

Biden had warm words about the alliance when he and Johnson met for the first time at a G-7 summit in England in June.

“We affirmed the special relationship — it’s not said lightly — the special relationship between our people,” Biden said.

Although the two leaders struck a positive tone when they met, they have very different temperaments. Biden staunchly opposed Britain’s exit from the European Union that was championed by the populist, crowd-pleasing Johnson, and once called him a “physical and emotional clone” of former President Donald Trump.

Britain’s lack of clout with Washington underscores the fragility of Johnson’s goal of making post-Brexit “Global Britain” a key international player and bridge between the U.S. and Europe.

Leaders across Europe were relieved when Biden replaced Trump, an isolationist who repeatedly disparaged NATO and insulted America’s allies. Biden renewed U.S. commitment to global climate change targets, which Trump had ripped up, and reassured allies that the U.S. was back as a reliable partner.

But he has stuck to Trump’s commitment to end the U.S. war in Afghanistan, even as the speed of the Taliban takeover this month caught Washington and its allies by surprise.

Opponents say Johnson's unwillingness to work closely with the EU and his decision to slash Britain's foreign aid budget, citing the economic blow of the pandemic, makes “Global Britain” little more than a slogan.

And they accuse the government of a slow and disorganized response to the Afghanistan crisis. Foreign Secretary Dominic Raab was on vacation in Greece as the Taliban swept toward the Afghan capital, returning a day after Kabul fell on Aug. 15.

Johnson spoke to Biden by phone on Aug. 17, two days after the fall of Kabul. In a bland statement after the call, Johnson’s office said they “resolved to continue working closely together on this in the days and weeks ahead to allow as many people as possible to leave the country.” British media reported it took Biden 36 hours to return the British leader’s call.

U.K. Armed Forces Minister James Heappey acknowledged that Britain would have liked “a more conditions-based” U.S. withdrawal, but he insisted the special relationship remains strong.

“Of course when you disagree with your closest friend it hurts, it causes consternation on both sides of the relationship," he told Sky News. "But absolutely nobody should think that the U.S and the U.K. have anything but the deepest and strongest of relationships.”

Thomas Gift, director of the Center on U.S. Politics at University College London, said events in Afghanistan “will make other Western powers more steely eyed about the reality that, even under Biden, U.S. leadership will pursue policies that it sees in its own interests, regardless of criticism by the international community.”

But he said Biden remained much more of an international team player than Trump.

“What we’re seeing here is not simply a diluted version of an ‘America first, America alone’ foreign policy,” he said. “We’re seeing Biden on one issue — granted, a hugely important one — reach a different conclusion than many other global leaders.”

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