NEW YORK (NYTIMES) - After gruelling years of watching American forces fight and die in a faraway land, the US president appealed to growing war weariness among voters and brought the troops home.

Not long after, an extremist group stormed through areas the Americans had left, killing civilians, seizing power and sweeping away billions of dollars' worth of US efforts to leave behind a stable nation.

That's what happened after President Barack Obama withdrew US forces from Iraq in 2011: The jihadis of the Islamic State group established an extremist emirate, prompting the United States to dispatch its military, yet again, to flush them out.

It is also now a possible scenario in Afghanistan, where President Joe Biden's order to shut down America's longest war has led to swift advances by the Taleban, the same extremist group the United States invaded Afghanistan to topple after the terrorist attacks of Sept 11, 2001.

The challenge of achieving US interests in complex and distant societies like Afghanistan and Iraq has bedeviled policymakers from both parties since President George W. Bush declared the "war on terror" nearly two decades ago.

In the years since, even how those interests are defined has swung wildly, driven at some times by a desire to spread democracy and human rights and at others by exasperation that costly efforts by the United States have borne so little fruit.

The result, according to some analysts and former US officials, is a perception among both friends and enemies that you can never guarantee how long the United States will stick around.

"In my experience, we just have a lack of strategic patience as a nation and as a government," said Mr Ryan Crocker, a retired US diplomat who served as ambassador to Iraq and Afghanistan. "Sadly, in the region, our adversaries have come to count on us not staying the course."

Mr Biden has decided the time has come to leave Afghanistan, despite the risk that future developments could suck the United States back in.

In a speech last month defending his policy, Mr Biden argued that it was not America's job to fix the country.

"We did not go to Afghanistan to nation-build," Mr Biden said. "It's the right and the responsibility of Afghan people alone to decide their future and how they want to run their country."

After two decades, he argued, keeping the troops deployed just a little longer was "not a solution, but a recipe for being there indefinitely."

That policy has come under pressure in recent days, as Taleban forces seized six provincial capitals and exposed the weakness of the Afghan forces meant to take over after the United States completes its withdrawal at the end of the month.

During their advance, the Taleban have been accused of using assassinations and bombings to subvert talks aimed at creating a power-sharing government. Rights activists fear they will reimpose restrictions on women, barring them from working and moving around independently. And security experts warn that terrorist groups like al Qaeda and the Islamic State could use Afghanistan to plot new attacks abroad.

Mr Biden has given no indication that he might change course, his position backed by polls suggesting that most Americans support the withdrawal.

But casting a shadow over the pull-out is the ominous precedent of Mr Obama's withdrawal from Iraq in 2011, when Mr Biden was vice-president.

At the time, al Qaeda in Iraq had been largely routed by the United States and Iraqi forces. But the Iraqi government was rife with corruption, its military was ill-prepared to ensure security, and its society was divided by sectarianism that was exacerbated by US-backed politicians.

Two years later, after taking advantage of the chaos of Syria's civil war to establish a foothold there, Islamic State jihadis roared back into Iraq, seizing cities and establishing a so-called caliphate.

Shocked by the group's violence and worried that it would inspire terrorist attacks around the world, the US military returned, at the head of an international coalition that worked with local forces to rout the jihadis.

"You cannot but compare the two cases," said Mr Harith Hasan, a senior fellow at the Carnegie Middle East Centre, of Iraq in 2011 and Afghanistan today.

Many factors that contributed to the Islamic State's rise are present in Afghanistan, he said, adding that policymakers would be naive to think that such problems would not eventually spill over borders.

"Even if the US wants to disengage, the rise of forces such as Isis, such as the Taleban, forces that are radical and able to destabilise the whole region, they will eventually affect US interests," Mr Hasan said, using an alternative name for the Islamic State.

The United States has been involved in Afghanistan since Mr Bush ordered an invasion in 2001 aimed at disrupting terrorist groups and toppling the Taleban government.

But al Qaeda and Taleban leaders escaped to neighbouring Pakistan, and in 2003 the United States announced the end of major combat operations and shifted, with international partners, to helping Afghanistan emerge as a pro-Western democracy.

Despite some successes, corruption in the Afghan government sapped development, neighbouring powers backed proxy forces inside the country, and the Taleban reconstituted as an insurgency.

Mr Obama increased and decreased troop numbers, and President Donald Trump started negotiations with the Taleban, bypassing the Afghan government.

After entering the White House, Mr Biden announced the withdrawal, arguing essentially that if all the US had done so far had not fixed Afghanistan, nothing would.

Many Americans share that view. Others worry it sets a dangerous precedent.

The US withdrawal after negotiating with the Taleban amounted to "an effective American surrender," said Mr Crocker, the former ambassador.

"The Taleban can now present themselves as the Islamic movement that defeated the great Satan, and that is going to resonate internationally," he said.

The speed of the withdrawal has left many Afghans feeling that they are being left alone to deal alone with messes created, in part, by US policies.

Professor Omar Sadr, a professor of political science in the Afghan capital, Kabul, attributed problems like the weakness of the Afghan security forces to the United States' failure to work inside the country's political and cultural dynamics.

"The entire process of state building was based not on local needs and circumstances but on a model that was brought here from outside," he said. "Now, we are seeing the rapid collapse of the state order."

Ms Orzala Nemat, an Afghan scholar and researcher, said that she had opposed the invasion, but that in the years since, the United States had become too enmeshed in Afghan life to pull out so suddenly.

"You made this country extensively dependent in every aspect for 20 years, and then one day you decide this is the time, and you leave without securing it to be able to make any progress," she said.

She acknowledged that many parties had contributed to Afghanistan's current troubles, but said that did not absolve the United States of its role.

"Because of the size, the speed and the scale of their involvement in Afghanistan, they have a larger share of responsibility," she said.