America looked back last week on 9/11 and its aftermath in Afghanistan with a mood of deep pessimism. The Taliban victory in Kabul seemed to end not just a 20-year war, but an era in American life.

This bleak assessment was shared in retrospective essays by two of journalism’s best commentators. “After 9/11, the U.S. Got Almost Everything Wrong,” was the title of Garrett M. Graff’s piece in the Atlantic. “9/11 was a test. We failed.,” read the headline on Carlos Lozada’s account in The Washington Post.

The consensus of these and many other appraisals, at once correct and incomplete, is that America went down the wrong road after the al-Qaida attacks. The country began with a rare unity of purpose, but this gradually dissipated – in part because of mistakes abroad and internal divisions. The flags displayed from nearly every house on 9/11 became the flags used as spears in the insurrection of Jan. 6.

Tony Blair, Britain’s former prime minister, sees a loss of “strategic will” in America’s withdrawal from Afghanistan. That’s too harsh. The basic mission of avenging 9/11 and shattering al-Qaida’s core leadership was achieved long ago. I didn’t agree with President Joe Biden’s decision to remove our small remaining force, but to a majority of Americans, the logic was pretty simple: If you’re in a hole, stop digging.

Radical pessimism is a mistake. The two decades since 9/11 witnessed many American blunders but also lessons learned. Our military commanders discovered how to project power at relatively low cost, in Iraq, Syria and Afghanistan. Despite the Taliban’s triumph, Islamist radicalism has been gradually on the wane – in Syria, Iraq, Saudi Arabia, the United Arab Emirates, Egypt and a half-dozen other places.

What’s indisputably true is that a cycle in U.S. history has ended.

I don’t just mean the post-9/11 effort to remake the Middle East by force. A larger process has been at work over the past century, as the United States gradually replaced the European colonial powers and took up their burden. This post-post-colonial era is dead, thankfully. The American people won’t stand for it anymore, and neither will the rest of the world.

Afghanistan may feel like a defeat, but it’s more like an armed retreat. Britain’s forays into these tribal lands in the 19th century were similar: They began with outrages that felt to Londoners like 9/11, and as you read histories of Britain’s misadventures, you realize that it’s the same story we’ve been living. British fusiliers and “light brigade” cavalry couldn’t subdue Afghanistan’s fiercely independent tribal fighters, and neither could our drones.

The British used to argue whether it was better to have what they called a “forward” strategy of engagement and invasion in places such as Afghanistan, or a more cautious standoff policy that was sometimes described as “masterly inactivity.” The hawks usually won that fight, following the ever-onward logic of Maj. Gen. Robert Clive, known as “Clive of India,” who pronounced in the 18th-century beginnings of empire: “To stop is dangerous, to recede is ruin.” That all-or-nothing rhetoric for America is a welcome casualty of Afghanistan.

On the 9/11 anniversary, we considered the endings of other modern wars, and the sense of exhaustion and uncertainty that followed. Certainly, that was true after World War I, where trench warfare destroyed not just millions of soldiers, but the confident, aristocrat-led culture that sent them to battle.

World War II brought more optimism but also anguish about how the slaughter of 6 million Jews and tens of millions of others could have happened. As Louis Menand reminds us in his superb new cultural history, “The Free World,” the triumph of 1945 was accompanied by horror over Nazi and communist totalitarianism’s assault on the individual. Post-1945 America was proud but also fearful.

With the Taliban back in Kabul just 20 years after 9/11, it’s tempting to see this story as inevitable. But it wasn’t.

We arrived at the present after a series of inflection points; accidents and choices that could have gone differently. The lights “blinking red” about al-Qaida’s plot could have been seen; Osama bin Laden could have been killed as he escaped into the mountains at Tora Bora in 2001; President George W. Bush might have listened to warnings from former national security advisers Brent Scowcroft and Zbigniew Brzezinski not to invade Iraq in 2003; a different American proconsul in Baghdad than Paul Bremer might not have disbanded the Iraqi army. The list goes on.

A decade ago, thinking about the U.S. war in Afghanistan, I came across a passage from “Paradise Lost” by the British poet John Milton: “Revenge, at first though sweet, bitter ere long back on itself recoils.” The language may be archaic, but after these 20 years, we know what it means.