When I taught American foreign policy, I always began my lectures on Vietnam by showing the class Lesson No. 9 from “The Fog of War,” Errol Morris’ penetrating documentary about former Secretary of Defense Robert Strange McNamara. The lesson? In order to do good, you may have to engage in evil. Undoubtedly, that contradictory logic has justified some of the United States’ most ferocious acts abroad. The nuclear attacks on Hiroshima and Nagasaki, and the bombing of North Vietnam, are two extreme examples.

Immediately after the clip ended, I would survey the 40-odd college students’ faces looking up at me for a conclusion. Without fail, some were disgusted, some were confused, and always, some were proud. Instead of answering them, I would ask, Are McNamara’s actions excusable?

It helps, in approaching that question, to know just what McNamara did. After he died Monday at the age of 93, the New York Times astutely summarized his professional life:

Mr. McNamara oversaw hundreds of military missions, thousands of nuclear weapons and billions of dollars in military spending and foreign arms sales. . . . Half a million American soldiers went to war on his watch. More than 16,000 died; 42,000 more would fall in the seven years to come. . . . He concluded well before leaving the Pentagon that the war was futile, but he did not share that insight with the public until late in his life.

That war, of course, was Vietnam.

In some regards, taking the moral stick to McNamara’s legacy 40 years hence is anachronistic. The Cold War is long over. Not only did the Soviet Union crumble 20 years ago, but with a haunting irony, on the same day that McNamara passed, President Barack Obama appeared at the Kremlin in Moscow beside Russian President Dmitry Medvedev to announce a nuclear weapons agreement that would reduce each country’s stockpiles by a third. The scene was light years away from the debates McNamara had within the Kennedy administration about the “missile gap” between the two nuclear superpowers—debates that foreshadowed decades of escalation.

In another regard, however, McNamara’s dilemma seems entirely apt today, even if hard-drawn analogies between Vietnam and Afghanistan are overblown. After seven years of combat, there were nearly 400,000 soldiers in Vietnam. After the same amount of time in Afghanistan, the numbers are heading for 68,000.

Yet, the similarities are more haunting. As in Vietnam, the end toward which U.S. forces in Afghanistan are now fighting remains unclear. Keeping the dominoes from falling in Southeast Asia turned out to be a dubious mission four decades ago. Today, the best and brightest foreign policy minds still wonder whether the mission in South Asia is to fight the Taliban or to build a nation.

McNamara didn’t waver. As secretary of defense from 1961 to 1968, he oversaw the escalation of the war in Vietnam. Privately, he may have had doubts. Publicly, and serving multiple presidents, he made policy decisions to support his superiors. Again, the similarity to recent events is obvious. Richard Haass, a former aide to President George W. Bush and now president of the Council on Foreign Relations, was recently questioned for not taking his internal opposition, in the run-up to the war in Iraq, public. As he explained, “Dissent tends to be more honored in the abstract than in practice.”

McNamara’s subsequent career move – president of the World Bank for more than a decade – suggests that he had realized the error of his ways long before he wrote the memoir that revealed his misgivings about the country’s imprudent war in Vietnam. In “The Fog of War,” he recalls a heartbreaking scene from early November 1965. A Quaker named Norman Morrison took up a position below the window of McNamara’s Pentagon office and, in protest of the war, lit himself on fire with a baby in his arms. Morrison’s wife later explained her husband’s self-immolation as a message that people had to stop killing other people. McNamara reflected in the film, “That’s a belief I shared. I believed it then, and I believe it even more strongly today.”

Reading the reflections – so many of them harsh and censorious – about McNamara’s life, it’s striking how unforgiving they are. Writing in the New York Times, Bob Herbert asked, “How in God’s name did he ever look at himself in a mirror? Lessons learned from Vietnam? None.” The former question is entirely useful. The latter declaration does us all a disservice.

I always introduced the Vietnam war to my students with Morris’ film because to dismiss McNamara out of hand as a war criminal, a monster or a failure as a human being leaves us little with which to understand the decisions we face today. Not even the best and brightest have the piercing intelligence, the infallible foresight or just the pure dumb luck to make the wisest choice in every situation. Certainly McNamara didn’t. But having made mistakes, at least he reflected on them, and later explained.

Beginning with empathy, in the way that Morris delivered it in his film, makes the lessons of Vietnam more compelling to a classroom full of young, skeptical and intelligent minds. Without it, they may write off the history of that war as idiotic, learning nothing of how it came to be. In which case, we will certainly be doomed to repeat it.