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 Mr. Biden even before he took office about the potential for the Taliban to overrun the Afghan army, but intelligence estimates, now shown to have badly missed the mark, assessed it might happen in 18 months, not 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. In the past several days, the Afghan forces have steadily collapsed as they battled to defend ever shrinking territory, losing Mazar-i-Sharif, the country’s economic engine, to the Taliban on Saturday…

“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,” he added, “but the Pentagon believed its own narrative that we would stay forever.”

“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?”