When President Joe Biden chose Sept. 11 as the deadline for U.S. forces to withdraw from Afghanistan, the date suggested a symmetrical coda to the 9/11 attacks that triggered the war 20 years earlier.

But nothing is neat about war, especially a conflict that raged for decades even before U.S. involvement. And more fighting is in store as the Taliban — the theocratic extremists who ran Afghanistan into the ground and gave succor to al-Qaida terrorists — are rapidly advancing, conquering districts or just accepting surrender from depleted, demoralized Afghan forces.

The Taliban hasn't overrun the country's capital, Kabul — yet. But U.S. intelligence estimates that the government could fall in as little as six months, and/or Afghanistan could revert to the kind of civil war, replete with warlords and militias, that spawned the Taliban. "That should be a concern for the world," Gen. Austin S. Miller, the top U.S. commander in Afghanistan, recently said.

Indeed, it should. So should any scenario that restores Taliban rule, which was particularly cruel to women and girls. It would further destabilize an already volatile region reeling from warfare and the pandemic.

The Taliban advances are likely to accelerate as the drawdown quickens. Last week saw the substantive and symbolic departure from Bagram Air Base, the hub of operations for the U.S. and several NATO nations who nobly stood by America after it invoked the alliance's collective-defense mechanism known as Article 5.

"We are on track, exactly where we expected to be," Biden said after the Bagram departure. Maybe so, as far as the drawdown goes. But it's not where Americans expected to be after initially vanquishing the Taliban and nurturing a fledgling Afghan democracy. Instead, the Taliban is in the process of an unlikely, unwelcome comeback that threatens the gains made by Afghan, U.S. and allied forces.

Biden is more inheritor than instigator of the drawdown, if not an eventual defeat. But it's happening on his watch, and as vice president he was part of an Obama administration that also made multiple mistakes on Afghan policy. Previous presidents of both parties did, too, starting with George W. Bush, who squandered success to turn to a different war, one of choice, not necessity, in Iraq. For his part, former President Donald Trump first talked tough on Afghanistan and then oversaw a deal that the Taliban is not honoring. He, too, set a drawdown date that would have led to similar results as are unfolding today.

But Afghan Presidents Hamid Karzai and Ashraf Ghani are far more to blame than American leaders, presiding over feckless efforts to build an effective military, let alone a cohesive society. And they allowed (if not engaged in) corruption that corroded the moral authority needed to govern.

Afghans must "decide their future," Biden said after recently meeting Ghani. And Americans have to decide how much they will still help to determine that future. Biden is right to prioritize the safety of those who directly helped U.S. forces, including interpreters. And the administration is taking some other steps to soften the drawdown's blow, if just a bit. But Afghans will mostly soon be on their own.

While there is no desire in Congress or the public to replicate the Obama-era "surge," Biden shouldn't have surged an expedited drawdown amid Taliban gains. And he and future presidents, as well as the American people, should heed the central lesson of Afghanistan as some in the public and Congress call for military options against Iran and other U.S. adversaries:

Entering "forever wars" is easier than exiting them.