A history of U.S. miscalculation is compounded by the lack of an exit plan.

If there is a consistent theme over two decades of war in Afghanistan, it is the overestimation of the results of the $83 billion the United States has spent since 2001 training and equipping the Afghan security forces and an underestimation of the brutal, wily strategy of the Taliban.

The Pentagon had issued dire warnings to President Biden even before he took office about the potential for the Taliban to overrun the Afghan Army. But intelligence estimates indicated that it might happen in 18 months, not within weeks.

Commanders did know that the afflictions of the Afghan forces had never been cured: the deep corruption, the failure by the government to pay many Afghan soldiers and police officers for months, the defections, the soldiers sent to the front without adequate food and water, let alone arms.

Mr. Biden’s aides say that the persistence of those problems reinforced his belief that the United States could not prop up the Afghan government and its military in perpetuity. In Oval Office meetings this spring, he told aides that staying another year, or even five, would not make a substantial difference and was not worth the risks.

In the end, an Afghan force that did not believe in itself and a U.S. effort that Mr. Biden, and most Americans, no longer believed would alter events combined to bring an ignoble close to America’s longest war. The United States kept forces in Afghanistan far longer than the British did in the 19th century, and twice as long as the Soviets — with roughly the same results.

For Mr. Biden, the last of four American presidents to face painful choices in Afghanistan but the first to get out, the debate about a final withdrawal and the miscalculations over how to execute it began the moment he took office.

“Under Trump, we were one tweet away from complete, precipitous withdrawal,” said Douglas E. Lute, a retired general who directed Afghan strategy at the National Security Council for Presidents George W. Bush and Barack Obama.

“Under Biden, it was clear to everyone who knew him, who saw him pressing for a vastly reduced force more than a decade ago, that he was determined to end U.S. military involvement,” Mr. Lute added, “but the Pentagon believed its own narrative that we would stay forever.”

He continued, “The puzzle for me is the absence of contingency planning: If everyone knew we were headed for the exits, why did we not have a plan over the past two years for making this work