If the purpose of the Afghan War was to prevent terrorists from using Afghanistan to stage attacks on the United States, America could have declared victory and gone home years ago. But if the purpose was also to help build a durable state—to prevent the Taliban from taking power again, destroying the hard-won progress made by millions of Afghan citizens, and perhaps allowing radical Islamists to regain a base on Afghan soil—then the longest war in U.S. history is ending in defeat. President Joe Biden’s announcement that all American forces will be withdrawn by September 11—a neat two decades after the attacks that propelled the U.S. into Afghanistan—simply gives a date after which the clock on the demise of the Afghan government will begin to tick.

The South Vietnamese government lasted a little more than two years after the final withdrawal of American troops, in January 1973, before Saigon fell to North Vietnamese forces in April 1975. Henry Kissinger, who negotiated the peace agreement between the U.S. and North Vietnam, once referred to this period as “a decent interval”—long enough for the abandonment of an ally to be obscured, for defeat to be laid at the door of the Vietnamese rather than the Americans. In Afghanistan the interval will be even less decent; the takeover by the Taliban after the withdrawal of American and other NATO forces will very likely happen much faster. We should assume that Afghan cities could fall within months or weeks and that Kabul will soon become a bloody battleground, as it was in the early 1990s, during the civil war that followed the departure of Soviet troops. We should also assume that the Taliban will be no more merciful toward women, girls, religious minorities, civil-society activists, political opponents, and perceived infidels and spies than during its years in power before September 11, 2001.

As I recently wrote, Biden has a relevant personal history. In April 1975, as a first-term senator, he was an outspoken opponent of using American money and risking Americans’ safety to rescue the tens of thousands of South Vietnamese who had bet their lives on American promises. “The United States has no obligation to evacuate one, or 100,001, South Vietnamese,” he said in a Senate speech. President Gerald Ford tried to sway Biden by reminding him of the American tradition of welcoming refugees from war and oppression, but Biden was unmoved. Vietnam was a lost cause, and Americans wanted to forget.

As South Vietnam fell, 135,000 endangered Vietnamese were evacuated through the heroic efforts of American officials, military veterans, and private citizens. Ford later said, “To do anything less would, in my opinion, only add moral shame to military humiliation.” Those refugees and their descendants are now Americans. I doubt that Biden would wish it otherwise.

Biden failed to see a moral obligation in 1975. Today he can learn from the mistake and redeem it. Seventeen thousand Afghans who have worked for America in Afghanistan, along with tens of thousands of their family members, are waiting for the excruciatingly slow bureaucratic wheels of the U.S. government to process their visa applications. At the normal pace, they will still be waiting years after the last American troops leave their country. While they wait, trying to hide, many of them will be hunted down by the Taliban. We will be gone, and Afghans who believed our promises will be killed. Our war will be over—Americans might not even hear the news of their deaths.