WASHINGTON — The conclusion of the Pentagon’s two-decade effort in Afghanistan lays bare the challenges facing U.S. diplomats and aid workers who remain behind, as a modest civilian force attempts to propel warring Afghans toward peace and protect advances for women without the support and reach provided by the military mission.

Current and former officials described an array of obstacles that a shrinking cadre of civilians in the bunkered U.S. Embassy in Kabul must navigate, with the COVID-19 pandemic and the specter of a possible diplomatic evacuation compounding the significant difficulties inherent to working in Afghanistan.

“In the absence of a military complement in Kabul, the task of the U.S. Embassy is made infinitely more complex, dangerous and difficult,” said Hugo Llorens, who served as the top U.S. diplomat in Afghanistan under presidents Barack Obama and Donald Trump.

The diplomatic challenges have come into focus after President Joe Biden’s decision to withdraw U.S. forces by the end of August, a move that has emboldened the Taliban, which has intensified its campaign to retake lost ground, and deepened fears that the Kabul government could collapse.

Already a growing list of countries, including France and China, have evacuated their citizens from Afghanistan. Peace talks between the Taliban and Afghan government show few signs of progress.

Biden this month defended his decision, saying Afghans must now defend their nation. He also vowed the United States would not abandon Afghanistan, making the diplomatic and aid mission — in particular, U.S. support for local security forces and the plight of women and girls — a central test of the president’s strategy.

“The complexity of the operations in Afghanistan are orders of magnitude greater than virtually anywhere else,” said a former senior official with knowledge of the mission in Afghanistan, where insecurity, the country’s landlocked position in central Asia, its geography, intense poverty, and tribal and ethnic division all contribute to the cost and difficulty of the newly solitary civilian mission. Like others interviewed for this report, this person spoke on the condition of anonymity to discuss ongoing policymaking.

In the 20 years since the United States launched its first airstrikes in Afghanistan, the work of the State Department, U.S. Agency for International Development (USAID) and other civilian agencies has been largely overshadowed by the military mission, massively larger in scale, manpower and funding.

At the height of the “civilian surge” that accompanied Obama’s troop increase in 2010, aid workers and diplomats were arrayed at consulates and outposts across the country in an effort to lay the conditions for lasting security gains.

Even after that high-water mark, diplomats in Kabul benefited from the network of military bases and the presence of military aircraft, both enabling more frequent visits to far-flung outposts to review conditions on the ground and providing an additional channel of information from troops in the field.

The U.S. nation-building effort, which once included ambitious plans to modernize infrastructure and transform the country’s economy, is littered with examples of failure. According to the Special Inspector General for Afghanistan Reconstruction, an independent oversight entity established by Congress, at least $19 billion of the $134 billion American taxpayers spent on security, development and humanitarian aid since 2002 was lost due to waste, fraud and abuse — and potentially much more.

The assistance also contributed to major improvements for Afghans, including substantial reductions in infant and child mortality, and an increase in female life expectancy from 47 to over 60 years.

Today, diplomats and aid workers operate out of the hulking embassy complex in Kabul, located at the edge of a fortified diplomatic and government zone. The embassy is a city unto itself, with offices, apartments, dining and workout facilities where some 1,400 Americans, part of a total workforce of 4,000, are confined for months a time.

The military departure means reduced mobility for officials overseeing an assistance portfolio that made Afghanistan the largest recipient of U.S. aid in 2019, according to Concern Worldwide, increasing the difficulty of ensuring taxpayer dollars are spent as intended. USAID expects to spend up to $500 million on Afghanistan assistance in 2021.

Patricia Gossman, associate Asia director at Human Rights Watch, said Afghan women are in danger of losing the advances in freedom and education they have achieved since 2001. “If things return to an all-out civil war among militias and Taliban forces, we fear a repetition of the atrocities of the early ‘90s,” she said.

USAID has said it is planning for a range of scenarios, from a deterioration of security to a possible peace agreement. Although USAID and the groups it partners with frequently work under difficult conditions, the agency acknowledged in a statement that its operations “may become constrained depending on levels of violence and how the peace process unfolds.”

Officials say they have learned to rely on local aid groups or the local staff of international groups to operate in the most dangerous areas and have already developed means — sometimes using satellites — to monitor programs from afar.

“It’s not an on-off switch. It’s a rheostat,” the former senior official said. “Just because you can’t do as much as you’d like to do doesn’t mean you can’t do anything.”

The State Department has built up greater expertise in operating in dangerous environments over the past 20 years, despite deadly examples of when conditions have been more perilous than believed, as occurred when four Americans were killed in a 2012 attack on the U.S. Consulate in Benghazi, Libya.

Another major challenge will be overseeing the more than $3 billion the United States is expected to provide annually to fund Afghanistan’s security forces — a sum amounting to three-quarters of the country’s security budget and whose success will be central to the government’s ability to survive.

U.S. watchdogs have repeatedly warned that widespread corruption — from ghost soldiers to drug trafficking to the diversion of tires and bullets - has posed a major obstacle to making local army and police effective. Now, with the dismantling of a NATO-led military command that monitored U.S. and NATO security aid, an embassy security office must track the massive aid portfolio.

Perhaps the biggest question hanging over the diplomatic mission at the U.S. Embassy in Kabul, with its unsettled history and uncertain future, is how long it can last.

In 1979, U.S. Ambassador Adolph Dubs was abducted and shot dead under mysterious circumstances. A decade later, Washington closed the embassy amid the Soviet withdrawal. It did not officially reopen until 2002. Over time, insecurity has made life more restrictive for diplomats. In 2011, Taliban-linked militants launched a major assault on the embassy from a nearby building.

Despite Biden’s promise to execute a full military withdrawal, about 650 U.S. troops will remain, split between the embassy compound and Kabul’s international airport, which now represents a potential lifeline for foreigners who may need to evacuate in a hurry. Some troops will operate counter-rocket systems known as C-RAMs, which each have Gatling guns that shoot 4,500 rounds per minute.

Overseeing the embassy security mission is Rear Adm. Peter Vasely, a two-star Navy SEAL officer who was slated to oversee Special Operations in Afghanistan before Biden called for a withdrawal.

“He was available. He had the experience,” Marine Gen. Kenneth “Frank” McKenzie, head of U.S. Central Command, said during a recent visit to Afghanistan. “I know and trust him.”

Vasely’s team will operate partly out of the old NATO military headquarters, which will belong to the embassy. Security at the compound is now provided by the State Department, McKenzie said.

While Biden’s plan envisions U.S. air power being used in the post-withdrawal period only to target groups such as al-Qaida and the Islamic State, McKenzie said any threats against the embassy or the Kabul airport could be an exception.

If security deteriorates rapidly, the U.S. military trains specialized forces for missions like temporarily seizing an airfield to enable evacuation flights. Or, in drastic circumstances, it could launch a noncombatant evacuation operation as was done in Saigon in 1975 and Mogadishu in 1991.

European partners in the NATO mission, nearly all of which have now withdrawn their military forces, have expressed concern about whether they can maintain their diplomaticoperations. Many have already downsized their presence, closing consulates outside of Kabul and reducing embassy staff.

“We can see the Taliban are stronger and stronger, and it’s not a very bright picture. We feel short of options,” one senior European official said.

“All embassies will reduce or fully withdraw,” the official said. While those that remain will retain security provided by contractors, they face the same problem, the official said: “What to do if there is an emergency?”

Europeans and other coalition partners depend on the United States to provide logistical support and a last line of defense. But Biden administration officials have made clear the mission of the remaining military contingent is to secure the American Embassy, not those of other nations.

Germany, which completed its troop withdrawal June 29, has not yet downsized its embassy. But, according to a senior German official, “whatever happens to the airport in Kabul will be hugely important to every embassy remaining.”

Many allies are growing nervous about Washington’s failure to finalize an agreement with Turkey to provide forces to protect the airport. U.S. and Turkish officials say they are close to a deal, pending agreement on the participation of additional nations and other issues, a senior defense official said.

Turkish President Recep Tayyip Erdogan has said he wants Hungary and Poland to contribute troops. U.S. officials have said Turkey also has held talks with Albania and Pakistan.

While the Taliban has not made a public comment on U.S. plans to retain 650 troops, nearly a quarter of the total Washington said it would withdraw, the group has warned that any foreign troops remaining will be fair game. That includes forces at the airport, it said in a recent statement.

“If Turkish officials fail to reconsider their decision,” the militants said, “the responsibility for all consequences shall fall on the shoulders of those who interfere in the affairs of others.”

Lamothe reported from Kabul. The Washington Post’s Julie Tate contributed to this report.