Could it be that two generations from now our grandchildren are going to be watching a documentary about the consequences of America’s retreat in the Middle East? That’s the question I am pondering this week on the 40th anniversary of the communist conquest of Indochina.

The event is being marked by the broadcast on PBS of Rory Kennedy’s documentary “Last Days in Vietnam.” It is a stunning scoop that recounts the desperate scramble — by our GIs, spies and diplomats — to save our Vietnamese friends as a communist army descended on our ally.

Ms. Kennedy has come up with film clippings and interviews that are unbelievably harrowing. They show Vietnamese clambering to get into the American embassy compound in Saigon and onto helicopters, to be ferried out to American warships standing off shore.

So crowded were these vessels — so fast were things collapsing — that helicopters were pushed into the sea to make room on the decks. Ms. Kennedy found a photo of a child whose mother dropped her from a Chinook to a rescuer below. “Last Days” illuminates like few other films the human consequences of America’s retreat.

Rory Kennedy showed courage in making the film. It was, after all, her martyred father, Sen. Robert Kennedy, who led the revolt in the Democratic Party against the Vietnam War, into which America had been led by her martyred uncle, JFK.

“Last Days” doesn’t get into that story (and no one faults Rory Kennedy for that). But there are those of us who were invested in Vietnam and who hunger for a new telling of the history of how we betrayed an ally in pursuit of a peace pact with a determined foe.

Particularly now, when we are once again in negotiations with, in Iran, a hostile regime that is maneuvering against, in Israel, a beleaguered American ally. All the more so because of the central role in both chapters of John Kerry.

After Kerry got out of the Navy, he actually met with the Viet Cong in Paris, while they were still making war against us. He acknowledged this himself when, in an appearance before the Senate, he testified against our own GIs.

In that testimony, he and a leading anti-war senator, George Aiken of Vermont, joked about the betrayal that was brewing. “Do you believe the North Vietnamese would seriously undertake to impede our complete withdrawal?” Aiken asked.

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

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

“I would say they would be more prone to do that than the army of the South Vietnamese,” Kerry responded. This time not only to laughter but also to applause in the face of the danger that in an American retreat millions would be doomed.

Could even Shakespeare have imagined such cynicism in the Senate of a country at war? Or that such a figure as Mr. Kerry could have risen in the Senate to chair the very foreign relations committee before which he once testified?

It is that very committee that has been maneuvering to gain congressional oversight of what President Obama and Secretary Kerry are doing with Iran. Mr. Kerry is proceeding in the face of statements like the one in March from an Iranian militia leader that the destruction of Israel is “non-negotiable.”

Which is why this anniversary has me thinking of Israel as well as Vietnam. One thing to remember about Vietnam is the communists started breaking the Paris accord from the moment the ink was dry. They never had the slightest intention of abiding by the pact.

Iran is the same. Israel understands this, even if Kerry and President Obama don’t. It is why Prime Minister Netanyahu was so determined to make his plea to Congress. It is why the leadership in Congress is seeking to assert its authority to oversee the Iran deal.

No one belittles America’s sacrifice in Vietnam. It was enormous. We gave 58,000 lives and billions in treasure. And we won the war militarily. Then we gave it up and let it all go — 40 years ago this week.

This column first appeared in the New York Post.