The prospect that John Kerry is going to make a second run for the presidency is in the news. This is owing not only to a series of interviews in which he pointedly holds open the possibility that he may challenge President Trump in 2020. It also owes to the fact that his new book — “Every Day Is Extra,” an autobiography — gives only glancing coverage to the seminal event of his political career.

That is the testimony against his fellow GIs that Mr. Kerry, having just gotten out of the Navy, delivered in 1971 to the Senate Foreign Relations Committee. These columns have covered it before, but if Mr. Kerry is going to run again it will beg for a new reprise. It is the moment in which Congress began the deliberations in which it eventually abandoned our ally, Free Vietnam, to the communists.

From beginning to end, Mr. Kerry’s testimony was an eloquent, impassioned pack of lies. That started with his attempt to adopt for his comrades in the anti-war movement the mantle of “winter soldiers.” It was a reference to Thomas Paine’s warning, in “The Crisis,” against the “summer soldier and the sunshine patriot,” who would “shrink from the service of their country.” Mr. Kerry covered himself in vainglory.

Mr. Kerry did serve in Vietnam, so it’s not that he shrank from appearing in arms. He did something worse. He turned on those who stuck with the fight. After emerging from the Navy, he went to Paris, where peace talks were underway, and met with, among others, enemy envoys. Then he brought their talking points back and began arguing for them here at home — including in his testimony before the Senate.

This is what has remained so horrifying about the event. It’s not just that Mr. Kerry accused his fellow GIs of committing war crimes. It’s that he argued the communist case. He relayed to the Senate the communists’ assurances that were we but to set a date for quitting Vietnam, they would permit our GIs safe passage out of the country. And the Devil with our allies in the free South Vietnamese republic.

To listen to this testimony in the cold light of history is nauseating. The key moment comes in an exchange with Senator George Aiken, an anti-war Republican of Vermont (and then the senior member of the whole Senate). Asked Aiken:

“Do you believe the North Vietnamese would seriously undertake to impede our complete withdrawal?”

“No, I do not believe that the North Vietnamese would and it has been clearly indicated at the Paris peace talks they would not,” Mr. Kerry replied.

“Do you think they might help carry the bags for us?” Aiken asked, provoking laughter.

“I would say,” sneered Mr. Kerry, joining in the joke, “they would be more prone to do that than the Army of the South Vietnamese.”

Here the transcript indicates that the hearing broke into laughter and applause — applause for abandoning our South Vietnamese allies, who had struggled for years to hold off a communist conquest, laughter at the prospect that by the millions they would either be cast into re-education camps or to take to the seas in rickety boats to escape a communist tyranny.

Mr. Kerry spoke briefly of the importance of us allowing “the South Vietnamese people to determine their own future.” They didn’t get that chance, of course. The idea that the Vietcong in the South were mounting an indigenous uprising was always a fiction. The North took over from them within hours of the conquest. And there is no democracy in Vietnam, even to this day.

No wonder Mr. Kerry gives but glancing mention of all this in his new book. In 1971, Mr. Kerry spoke to the Senate about his determination “to undertake one last mission, to search out and destroy the last vestige of this barbarous war.” When he ran in 2004, the veterans of the Swift Boat Veterans for Truth stopped him cold. It’s fourteen years latter and Mr. Kerry still can’t face up to the truths they told.