Photograph by David Hume Kennerly/White House via AP

Almost forty years ago, in April of 1975, as the North Vietnamese Army was sweeping through South Vietnam toward Saigon, President Gerald Ford addressed a joint session of Congress. He asked for seven hundred and twenty-two million dollars in emergency military assistance for the government of South Vietnam. He invoked the dire risk faced by tens of thousands of South Vietnamese, including those affiliated with the United States. In “Last Days in Vietnam,” Rory Kennedy’s gripping new documentary about the fall of South Vietnam and the chaotic U.S. evacuation, Henry Kissinger, who was the Secretary of State, says of Ford, “He had two major concerns. The first was to save as many people as we could. He cared for the human beings involved—that they were not just pawns and, once they had lost their military power, they were abandoned. The second was the honor of America—that we would not be seen at the final agony of South Vietnam as having stabbed it in the back.”

It’s a little jarring to hear Kissinger distance himself on moral grounds from using human beings as pawns. His and Richard Nixon’s policy in Southeast Asia amounted to little more than that: sacrificing untold hundreds of thousands of Vietnamese and Cambodians, as well as thousands of American troops, to bloodless terms like "credibility," "strategic realignment," and “peace with honor.” But Kissinger’s account of American efforts during the fall of South Vietnam is accurate: in Washington and in Saigon, officials went to great, though tragically belated, lengths to rescue those Vietnamese associated with the governments of South Vietnam and the United States. “Last Days in Vietnam” will unsettle many of your fixed ideas about the end of the war. (For example, the film raises the possibility that, had Nixon not resigned over Watergate, nine months before the fall of Saigon, the North Vietnamese wouldn’t have invaded the South so readily, because they regarded Nixon as a madman capable of anything.)

In the long view of history, the war was unwinnable. As Neil Sheehan’s masterpiece “A Bright Shining Lie” shows, it was a war of Vietnamese nationalism, and the French and American interventions were seen by most Vietnamese as last stands of colonialism rather than as Cold War imperatives. By that April, two years after the signing of the Paris Peace Accords and the withdrawal of the last American combat forces, most people back home didn’t want to hear the name of the country where, in twelve years, almost sixty thousand U.S. troops had died. Congress, reflecting that exhaustion, voted down Ford’s emergency request (which would only have postponed defeat). Hearing the news, the mild-mannered President cursed, “The sons of bitches!” The fall of Saigon was just days away.

“Last Days in Vietnam” is full of dramatic tales illustrated by vivid archival footage. With no space for a landing, a South Vietnamese pilot drops his family out of his transport helicopter, onto the deck of an offshore American Navy vessel, then dives into the South China Sea and saves himself as the chopper crashes into the waves. A Vietnamese student named Binh Po buys and talks his way onto the grounds of the U.S. Embassy in Saigon, joining ten thousand other desperate people, only to wind up among the four hundred and twenty left behind when an order from President Ford ends the evacuation prematurely and the last Marine chopper takes off. (Binh Po spent a year in a Communist reëducation camp before escaping from Vietnam by boat, in 1979.) Marine Sergeant Mike Sullivan and other Embassy guards, without orders, take it upon themselves to make sure that the Vietnamese they know personally—tailors, cooks, dishwashers, and their families—make it out on the Chinooks. As North Vietnamese tank divisions roll toward Saigon, individual Americans break official rules and risk their lives to get as many of the Vietnamese who worked with Americans as they can to safety, along with their families—an inspiring example of moral heroism in the final days of a war best known for its mistakes, crimes, and sheer waste.

At the same time, the evacuation was a disaster. Ambassador Graham Martin, a rigid Cold Warrior out of “The Quiet American,” refused to believe that Saigon was about to fall, and wouldn’t allow fixed-wing air evacuations from the Tan Son Nhut airbase while it remained out of North Vietnamese hands. The result of Martin’s delusion was the frantic helo lifts from the Embassy grounds, the last and worst option, too little and too late, which left tens of thousands of our Vietnamese allies behind to suffer the brutality of the North. Yet even Martin, who lost his only son in the war, emerges, more ambiguously, as a conscientious diplomat at the last hour, postponing his own evacuation long enough to get thousands of Vietnamese out.

Army Captain Stuart Herrington, one of the heroes of the evacuation, had to lie to the Vietnamese left behind at the Embassy, telling them that a big chopper was on the way, then sneak away to board the last flight off the roof. Still haunted, he speaks for the film: “The end of April of 1975 was the whole Vietnamese involvement in a microcosm. Promises made in good faith, promises broken, people being hurt because we didn’t get our act together. The whole Vietnamese war is a story that kind of sounds like that. But, on the other hand, sometimes there are moments when good people have to rise to the occasion and do the things that need to be done, and in Saigon there was no shortage of people like that.”

Back in 2007, when I started writing about the betrayal of Iraqis associated with America in Iraq, I spoke with two of the men featured in “Last Days in Vietnam”: Frank Snepp, the chief C.I.A. analyst in Saigon and the author of “Decent Interval,” an account of that period; and Richard Armitage, a naval officer, who returned to Vietnam as a civilian defense official and ended up bringing twenty thousand Vietnamese out on boats. Hearing their stories, I thought that the analogies with Iraq were obvious—willful blindness at the highest levels, no plan for rescuing Iraqis—but the differences were even sharper. The Vietnam-era Americans came off much better. With a few exceptions, it was hardly possible to imagine Embassy officials or troops in Baghdad taking great risks to get their Iraqi contacts out before we left. Relationships with Iraqis were much more distant, and Americans much more isolated, owing to security restrictions and other factors. Above all, in Baghdad there was a pervasive air of deskbound caution, buck-passing, and ass-covering, in contrast with the Wild West atmosphere that broke out, for better and for worse, in Saigon in April of 1975. It was all too easy for Americans in Iraq not to know what they didn’t want to know.

On Wednesday night, President Obama will speak to the country about his strategy for fighting the Islamic State in Iraq and al-Sham. I wonder if he’ll have a chance to see “Last Days in Vietnam,” which opened on Friday. He would probably be struck by the historical irony that, like Ford, he must try to explain to Congress and a weary, sour public why the U.S. should get involved again in a far-off, supposedly concluded war that most Americans now view as a waste.