Is 20 years long enough for a war we could never win?

President Biden apparently thinks so, which is why on Tuesday his administration announced that all U.S. troops will leave Afghanistan by September 11, the anniversary of the attacks that produced America’s longest war.

Though he won’t say it this way, this is an admission of America’s defeat. And it’s long overdue.

The big question, however, remains just how deep this concession really runs. If Biden and his foreign policy thinkers are willing to bow to the inevitable in Afghanistan, what does this mean for our posture in the global war on terror?

The defeat in Afghanistan has many parents. Biden is the fourth president to preside over this futile war. And while there’s plenty of good and bad in the way the military and civilian leaders carried it out, at bottom they were attempting the impossible, and inevitably failed.

Much of the time in politics and government, when things don’t turn out the way we want, wise people claim that if only we had listened to them, everything would have gone better. Others tell us we now know what might have worked, but only in hindsight.

But this is one of those unusual situations in which nobody can say that it all would have worked out if a different plan had been followed. That was the whole problem: There was no better plan. There were only different varieties of losing.

Some people understood that even in 2001 — but not very many. In the aftermath of the 9/11 attacks, with the knowledge that the Taliban had allowed al-Qaeda to operate from within their country, support for an invasion was overwhelming; this Gallup poll taken just after the conflict began showed Americans supporting the use of ground troops by a margin of 80-18.

To so many it felt righteous and triumphant: We’d strike back at those who struck us, and no one could say we weren’t justified. Those who warned we’d be pulled into the same quagmire that mired the Soviet Union there for nine years were dismissed as naive and unpatriotic. We’re smarter and more skilled, so of course we’d succeed.

So here we are 20 years later, having spent trillions of dollars there and lost thousands of lives, and for what? Our departure will make it possible if not likely that the Taliban will retake control of the country, imposing their brutality on a populace that has endured so much suffering. But if that happens, it will happen whether we go now or wait a year or five years or ten.

Biden and his brain trust appear to be admitting to this fundamental truth. A senior administration official told The Post that withdrawal by September is inevitable, and will not be “conditions-based,” because making this contingent on conditions “is a recipe for staying in Afghanistan forever.”

Translation: Nothing, nothing, nothing can turn this war into a good outcome for the United States.

What remains to be seen is how far this will go. Several other areas loom as tests.

First, there’s our drone program. Biden has ordered temporary limits on how drones can be used against suspected terrorists, pending an assessment of what this has produced, whether the legal basis for it is sound, and how more transparency around the program can be achieved.

The drone program has been a major component of our ongoing war on terror, so the quality of the evaluation, and whether a real change in policy takes place, will help clarify whether we’re fundamentally reorienting our posture.

Second, there’s the question of whether Biden will stop the chronic weakening of Congress’ authority to declare war long perpetrated by presidents of both parties. The administration recently pledged to work with Congress to repeal the open-ended authorizations that lawmakers gave the president during the George W. Bush years, which have been stretched to absurdity to justify military action since.

This, too, is a major feature of our “forever wars.” And so delivering meaningful change would entail Biden working with Congress to create much more narrow legal justifications for future action against terrorism, justifications that can’t be abused.

As Jack Goldsmith and Samuel Moyn have noted, Congress could affirmatively bar unilateral presidential military action except in the most circumscribed emergency situation. If Biden seriously is willing to see something like this happen, congressional Democrats will do it.

Biden set a very broad goal for himself, when he said the following back in 2018: “It is past time to end the forever wars, which have cost the United States untold blood and treasure.”

But truly ending the forever wars requires a deep change in mind-set, says Stephen Miles, executive the director of Win Without War.

“The Afghanistan news is a real first step in turning candidate Biden’s promises into President Biden’s policies,” Miles tells us. “The real test is, Will this news be the beginning of a more fundamental shift, which leaves behind this mind-set that we can bomb and kill our way out of the challenge of terrorism?”

Biden will be criticized in some quarters for finally doing what Bush and Obama and Donald Trump all wanted to do but didn’t — presuming he goes through with it. But sometimes, a president has to do what’s unpleasant but necessary, even if no one will throw him a parade for it.

And we’ll all be better off if we admit what should have been clear from the beginning: Taking control of Afghanistan was a terrible mistake, and we should have ended this interminable war long ago. Let’s hope the lessons learned run deep.

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