BRUSSELS (AP) — At its start, America’s war in Afghanistan was about retribution for 9/11. Then it was about shoring up a weak government and its weak army so that Osama bin Laden's al-Qaida could never again threaten the United States.

Now it's about over. With bin Laden long since dead and the United States not suffering another major attack, President Joe Biden is promising to end America’s longest war and move on to what he believes are bigger, more consequential challenges posed by a resurgent Russia and a rising China.

Even so, by withdrawing the remaining few thousand U.S. troops in Afghanistan by the 20th anniversary of the 9/11 attacks, Biden is taking a calculated risk that extremists in Afghanistan can be countered by U.S. and partner forces elsewhere in the region — and that he won’t become the president who underestimated the resilience and reach of extremists who still aim to attack the United States.

CIA Director William Burns told Congress on Wednesday the U.S. unavoidably will lose some intelligence leverage against the extremist threat, although he suggested the losses would be manageable.

“The U.S. government’s ability to collect and act on threats will diminish. That’s simply a fact,” Burns said. "It is also a fact, however, that after withdrawal, whenever that time comes, the CIA and all of our partners in the U.S. government will retain a suite of capabilities, some of it remaining in place, some of them that we will generate, that can help us to anticipate and contest any rebuilding effort.”

There were 2,500 to 3,000 U.S. troops in Afghanistan when Biden took office, the smallest number since early in the war. The number peaked at 100,000 during President Barack Obama’s first term. As U.S. war casualties have declined, so has the American public’s attention. The war was barely mentioned during last year’s presidential contest, and pulling the plug may prove politically popular.

Yet worries remain. Stephen Biddle, a Columbia University professor who has advised U.S. commanders in Afghanistan, says it's possible al-Qaida could re-establish its base structure in Afghanistan once the Americans and their coalition partners leave. The Taliban in Afghanistan pledged in a February 2020 agreement with the Trump administration that they would not allow al-Qaida or other extremist groups to use Afghan territory to threaten the United States. But that deal may be imperiled by Biden's decision not to complete the withdrawal of forces by May 1, as the Trump administration had promised.

The bigger peril, Biddle said in an email exchange, is that the withdrawal could lead to the collapse of Afghan security forces and multi-sided civil warfare involving Taliban factions and others "in a more-lethal version of the civil war of the 1990s.”

“This would be a humanitarian disaster for Afghans — far worse than today’s insurgency," he said.

More broadly, the absence of U.S. forces in Afghanistan could lead to further instability in a region with two rival nuclear powers — Pakistan and India, which have insurgencies of their own to contend with.

“This is already a dangerous part of the world; making it worse by allowing the collapse of the Afghan government is the biggest risk here,” Biddle said.

At a previously pivotal moment in the war, Obama took a similar view. When he announced a surge of 30,000 U.S. troops to Afghanistan in December 2009, he argued against trying to contain extremist threats in the Afghanistan-Pakistan region only with what the U.S. military calls “over-the-horizon” forces — troops and aircraft positioned beyond Afghan borders.

“To abandon this area now — and to rely only on efforts against al-Qaida from a distance — would significantly hamper our ability to keep the pressure on al-Qaida and create an unacceptable risk of additional attacks on our homeland and our allies,” Obama said.

So Obama went ahead with a troop buildup aimed at hitting the Taliban so hard that they would agree to negotiate a peace deal. It didn't work. The Taliban kept fighting. Even after President Donald Trump authorized a more muscular military approach to the Taliban in 2017, the hard-hit militant group did not give up. It agreed to negotiate with the Afghan government, but those talks have stalled.

It's difficult to judge what has been gained in the 12 years since Obama escalated the war. Afghan security forces likely are stronger, although their resilience will be tested in the absence of U.S. support they grew to rely upon. The Afghan government has not strengthened its authority across the country, and the Pentagon argues that its intense focus on countering insurgents there and in the Middle East has been such a drain on resources that the U.S. is losing ground against China and Russia.

The war has cost more than 2,300 U.S. lives and immeasurable suffering among Afghans since the United States invaded in October 2001. Ten years into the war, in May 2011, U.S. forces killed bin Laden in Pakistan, and for a short time it seemed possible that Washington would see an opening for ending the war.

A few weeks after bin Laden's death, a young American soldier at a dusty outpost in eastern Afghanistan asked visiting Defense Secretary Robert Gates what effect the al-Qaida leader's demise would have on the war, suggesting hope that it would hasten its end and allow troops to go home.

“It is too early to tell,” Gates replied.

Ten years later, Biden has decided the time has come, although for Afghans the war may be far from over.

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Robert Burns has reported on the war in Afghanistan since the 2001 U.S. invasion and has covered national security issues from Washington since 1990.

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AP writer Eric Tucker contributed to this report.