A recurring motif in the history of America’s military engagements is that when US forces are withdrawn too soon, the consequences tend to be tragic for the people they leave behind.

Because Union troops didn’t stay long enough in the defeated South after the Civil War, Reconstruction was prematurely aborted and the freed slaves’ political rights were brutally extinguished. Because the United States withdrew its troops from Indochina in 1973, South Vietnam and Cambodia were helpless against the communist conquest and totalitarian nightmare that followed. Because Barack Obama in 2011 pulled all US forces out of Iraq, the monstrous Islamic State was able to overrun a vast swath of the country and fill it with horror and blood.

By contrast, because US troops remained in Japan, Germany, and Italy following World War II, all three reaped the blessings of peace, prosperity, and stable democracy. The same was true of South Korea, where a large American military presence remains nearly seven decades after the Korean War was fought to a stalemate.

In announcing last week that all remaining US troops will be withdrawn from Afghanistan by Sept. 11, President Biden declared that “it’s time to end the forever war” — a war, he said, that “was never meant to be a multi-generational undertaking.” By that argument, why not walk away from South Korea? Why haven’t we washed our hands of that “forever war”? The answer is that to leave would be dangerous and destabilizing, imperiling not only the people of South Korea but US strategic interests as well.

The same is true of Afghanistan.

If the United States abandons Afghanistan, it is virtually certain that the Taliban will grow stronger and bolder. Without America’s moderating influence, civil war is certain to escalate, the shaky and inept — but democratic — government in Kabul may be toppled, and Afghanistan is likely to again become a sanctuary for al-Qaeda. In fact, warns the Afghanistan Study Group, a bipartisan commission chartered by Congress, “a precipitous withdrawal could lead to a reconstitution of the terrorist threat to the US homeland within 18 months to three years.” In other words, if Biden carries out his determination to withdraw every American service member this year, jihadists in Afghanistan could be organizing a new 9/11 by the spring of 2023.

The president is getting similar warnings from his military and intelligence advisers.

According to The Washington Post, the chairman of the Joint Chiefs of Staff and senior commanders have told Biden that a pullout is likely to trigger “the fall of Kabul to the Taliban, waves of Afghan refugees rushing to neighboring countries and Europe, and the reemergence of al-Qaeda as a potent terrorist threat.” The new 2021 Threat Assessment from the Director of National Intelligence is similarly bleak about Afghanistan’s prospects if America cuts and runs: “The Taliban is likely to make gains on the battlefield, and the Afghan government will struggle to hold the Taliban at bay if the coalition withdraws support.”

America’s 20-year involvement in Afghanistan has not transformed the country into a modern liberal democracy. It has, however, made possible an immense improvement in the lives of countless Afghan civilians, who were rescued from the Islamist hell of Taliban rule and given a chance at a decent life. Especially dramatic has been the improvement in the status of women. Before the Americans arrived, girls were forbidden to attend school. Women were barred from public life and could be flogged if they were seen outdoors without being covered from head to toe.

If US troops decamp from Afghanistan, the risk of disaster is enormous. What does Biden hope to win that can justify such a gamble?

Perhaps he foresees rich political rewards for the president who can claim to have extracted America from a seemingly endless war. That’s what Obama thought when he ordered the US out of Iraq — he boasted about it endlessly during his reelection campaign — but it turned out to be one of the worst blunders of his presidency. In the end, Obama was forced to send thousands of US troops back to Iraq.

Despite the talk of a “forever war,” US troops in Afghanistan are not embroiled in warfare. The last American combat death occurred well over a year ago. Only about 3,500 US military personnel are deployed to Afghanistan, and they are involved primarily in training and logistics. Removing those forces will gain America very little. But it will immeasurably benefit the Taliban, who will exult, with reason, at having driven out the Americans. Biden may be tired of the mission in Afghanistan. But is he really prepared to walk away and let Kabul fall to our enemies?

(Jeff Jacoby is a columnist for The Boston Globe).