About President Biden’s decision to retreat from Afghanistan we were just sitting down to write when an email arrived with the subject line “Amazing Thread.” It linked to a chat on Twitter quoting Richard Holbrooke’s account of a conversation he had with the then vice president, illuminating what lesson for Afghanistan the future 46th President drew from our abandonment of South Vietnam.

The conversation is reported in the book “Our Man,” by George Packer. He quotes Holbrooke, then special representative for Afghanistan, as relating an encounter with Mr. Biden in the White House. “When I mentioned the women’s issue, Mr. Biden erupted. Almost rising from his chair, he said, ‘I am not sending my boy back there to risk his life on behalf of women’s rights, it just won’t work, that’s not what they’re there for.’”

Mr. Holbrooke tried to outline the position he and Secretary of State Clinton had taken. Mr. Packer quotes Mr. Holbrooke as relating that Mr. Biden “thought it was bullshit, and this spiraled into a much larger discussion concerning the whole course of what would happen, and this was quite extraordinary. Joe took the position, plain and simple, that we have to get out of Afghanistan.”

Mr. Biden, in the Packer/Holbrooke telling, warned that “we’re facing a debacle politically” and that along with unemployment “Afghanistan was the other issue that could pull us down and we have to be on our way out, that we had to do what we did in Vietnam.” This, Mr. Packer quotes Holbrooke as saying, “shocked me and I commented immediately that I thought we had a certain obligation to the people who had trusted us.”

“Fuck that, we don’t have to worry about that,” Holbrooke quotes Mr. Biden as saying. “We did it in Vietnam, Nixon and Kissinger got away with it.” When Holberooke said, “But there are larger strategic consequences here,” Mr. Biden demanded, “What are they?” Holbrooke tried to outline them and related that Mr. Biden “clearly thought I was mouthing some kind of right-wing crap.”

There it is. What Mr. Biden appears to have been talking about was the betrayal of South Vietnam, when Mr. Nixon threatened President Thieu. The doughty South Vietnamese leader knew that the Peace Accord just struck with the communists at Paris would doom his country and balked at signing. Nixon sent him a letter, drafted by Kissinger, warning that South Vietnam would be abandoned by the American Congress.

“Brutality is nothing,” Nixon told Mr. Kissinger. “You have never seen it if this son-of-a-bitch doesn’t go along, believe me.”

Yet there is a big difference, at least in our view, between how President Nixon and Secretary Kissinger acquitted themselves in respect of Vietnam and what President Biden and the Democrats are fixing to do in Afghanistan. Mr. Kissinger believes, as do we, that America might have been able to sustain a free government in Vietnam had Nixon not been ousted in Watergate (dooming Vietnam was Watergate’s result, if not its purpose).

Even at the end of the Republican administration, President Ford and Secretary Kissinger were desperately working Congress in hopes of getting it to resupply the Vietnamese. The Democrats, in control of Congress, did cut off aid to South Vietnam. In the greatest tragedy since the loss of Eastern Europe, Vietnam fell, and Indochina was cast into the long night of communism.

Will Mr. Biden and Secretary Blinken be working Congress to try to save the day the way President Ford and Secretary Kissinger ended up trying — vainly, it turned out — to do in respect of Vietnam? And will they meet with the kind of rebuff the earlier generation got from the Democrats? The lesson Mr. Biden seems to have taken from Vietnam is that we can abandon our erstwhile allies, as Russia and China watch and the devil takes the hindmost.