AMERICA’S EVACUATION of Afghanistan is entering its final days. President Joe Biden has said that he wants all American forces out by August 31st. Some American allies such as France, Netherlands and Turkey have wound up flights. American troops are beginning to depart, too. What will the final hours of the airlift look like? “It’s usually quite easy to go in,” says Ben Barry of the International Institute for Strategic Studies, a think-tank, who once wrote a study of evacuations for Britain’s defence ministry. “But getting out, particularly if you can only get out by air, and you’re potentially surrounded by hostiles—that’s where it’s really difficult”.

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Consider the experience in Vietnam. When on April 29th 1975 President Gerald Ford ordered the final evacuation of Saigon by helicopter, more than a thousand American citizens were in the city. Bing Crosby’s rendition of “White Christmas” played on Armed Forces Radio, the signal for evacuees to gather at designated points around the city to board buses. They were taken to Tan Son Nhat Airport or the nearby American embassy, where marines had torn down trees in the car park to land helicopters. On the final two days, 7,800 people were flown to aircraft-carriers. America’s ambassador left the city at dawn with the embassy’s folded flag under his arm and nearly five hundred Vietnamese left behind in the compound. Behind him, the crowd outside broke through the gate and poured in.

That underscores the central challenge: what military planners call a “collapsing perimeter”. Currently, Kabul’s airport is patrolled by thousands of American troops and, sometimes only metres away, Taliban fighters. That has prevented a repeat of the chaotic scenes that saw Afghans swarm the runway, some clinging to moving planes, on August 16th. But as more troops leave, it becomes ever harder to guard the perimeter. “The moment the last 60 troops are in the last flight,” says Mr Barry, “it’s extremely vulnerable.”

The American army’s field manual for evacuations says that “one useful technique…is to evacuate all but one deployed rifle company, and to have sufficient aircraft or ships available at one time to extract this force in one lift.” One way to protect that vestigial force is with air power. America has a formidable array of gunships and warplanes circling Kabul. It has already used helicopters to drive crowds away from the runway by blasting them with air. More planes could be sent from bases in the Persian Gulf or an aircraft-carrier in the Arabian Sea. But firepower is of little use against large crowds of civilians. In practice, America cannot shoot its way out. It will have to rely on the co-operation of the Taliban.

Jonathan Shaw, a retired major-general who oversaw Britain’s withdrawal from Basra, Iraq, in 2007, says it was done peacefully because it was co-ordinated with local forces. “Even though we knew they were riddled with informers,” he says, “we judged that it was in their interests to avoid a bloodbath”. To that end, American generals have kept in touch with Taliban officials in Doha, Qatar. On August 23rd Bill Burns, the CIA chief, met the Taliban’s first deputy leader in Kabul.

America will be especially keen that the Taliban identify and check any threats from Islamic State’s local branch, which is targeting the airport. Such an attack took place on August 26th, when two explosions occurred close to the airport’s gates. The worry is that more may follow, heightening the chaos around the airport. “The last American troops are going to clamber on that plane and the crowd control will be left to the Taliban,” says Mr Shaw. “That will be a very interesting handover.”

Extracting troops is not the only issue, though. Having seen the Taliban seize and operate a large cache of American equipment and weaponry in recent weeks, the Pentagon will be keen to destroy anything valuable left behind on the airfield. But valedictory air strikes would not only pose a risk to civilian crowds; they would also infuriate the Taliban, who could retaliate by blocking the emigration of the numerous Afghans who once served Western militaries but could not get out in time. Leaving booty behind comes with its own cost, though. “The commander…must recognize the propaganda value this precipitous action has for an observer,” says the American field manual, “who can then say the United States withdrew in disorder.”

Dig deeper:
Afghanistan’s evacuation deadline is a deadly countdown
How will the Taliban rule this time?
America may pay dearly for defeat in Afghanistan