Everything Pope Francis does seems unprecedented and unmatched, but when he stands before the General Assembly of the United Nations this week, the image of another pope will intrude. The U.N. speech delivered by Pope Paul VI in 1965, almost exactly fifty years ago, resonated across the world. Closer to home, the Pope’s words opened a breach between me and my father, and though our situation was particular, our sad conflict was not unique, but typical of a generation. For people of a certain age, Paul VI at the U.N. remains the unlikely measure of the difference a pope can make, of the pain that can come when a pope speaks the truth, and of the tragedy that can follow when that truth is ignored.

Dad had anxiously awaited the papal message less because he was a devoted Catholic—he had studied for the priesthood, as I was doing at the time—than because of his job at the Pentagon. The founding director of the Defense Intelligence Agency, Lt. Gen. Joseph F. Carroll was expecting Pope Paul VI to stoutly affirm the recently launched American war in Vietnam. After all, during long service in the Vatican Secretariat of State, Giovanni Battista Montini had been at the elbow of the fiercely anti-Communist Pope Pius XII, and could be expected to grasp the urgency of holding the line against Communist aggression in Southeast Asia. Indeed, the Pope’s host in New York, the all-powerful Cardinal Francis Spellman, was himself an ardent supporter of the U.S. crusade in Vietnam.

It was October. The previous February, Lyndon Johnson, fresh off his victory over Barry Goldwater, whom he portrayed as a warmonger, had ordered the start of a massive air war against North Vietnam: Operation Rolling Thunder. In March, the first U.S. combat troops were deployed, and by the time of the Pope’s visit, Secretary of Defense Robert McNamara, presumably acting on assessments provided by the Defense Intelligence Agency, had set U.S. force levels at more than 100,000 troops. But the first stirrings of opposition to the war had come that spring and summer, and by October protests were building in dozens of cities around the world. The Pope’s support at the United Nations would underscore the war’s moral legitimacy—just in time.

But that’s not what happened. Paul VI summed up the whole purpose of his trip to America with words that astonished his listeners and the world:

Here our message reaches its culmination…These are the words you are looking for us to say and the words we cannot utter without feeling aware of their seriousness and solemnity: never again one against the other, never, never again! Was this not the very end for which the United Nations came into existence: to be against war and for peace?

Before arriving at the United Nations, Paul VI had met briefly with President Johnson at the nearby Waldorf Astoria Hotel, and one imagines Johnson wincing at what the Pope said next:

Listen to the words of a great man who is no longer with us, John Kennedy, who proclaimed four years ago: ‘Mankind must put an end to war, or war will put an end to mankind.’ There is no need for a long talk to proclaim the main purpose of your Institution. It is enough to recall that the blood of millions, countless unheard-of sufferings, useless massacres and frightening ruins have sanctioned the agreement that unites you with an oath that ought to change the future history of the world: never again war, never again war!

The pope’s hands were open as he spoke, as if in pleading. “It is peace,” he declared, using a word that was already taking on a perilous new resonance, “peace that has to guide the destiny of the nations of all mankind.”

Later, sitting in the living room of my parents’ house on General’s Row at Bolling Air Force Base in Washington, I asked my father what he thought of the Pope’s speech. My father was the best man I knew. It’s not too much to say that my impulses toward worship extended to him. My voice may have cracked with its question.

An orderly had just served my father his drink. He looked up from it. His collar was open, his blue tie loosened. He knew exactly what I was asking. He replied evenly, “His Holiness said ‘Drop your weapons.’ But in the real world our weapons keep the peace.”

“But Vietnam—“ I began.

Dad cut me off sharply. “His Holiness does his job by holding up the ideal. We do ours by seeing how it applies in a specific situation, which assumes information and expertise His Holiness doesn’t have.”

My father’s information and expertise were no doubt essential to the memo McNamara sent to Johnson a little over a month later, a recommendation to vastly escalate the number of U.S. combat troops, up to more than 400,000 in 1966, with 200,000 to follow in 1967. The depth of the American abyss was set. So much for the wisdom of “the real world.” Our nation’s tragedy would be the writ-large of my father’s.

Now another pope, standing before the U.N. General Assembly, can be expected to “hold up the ideal” on questions ranging from climate change to economic inequality to the plight of migrants. And once more, the possessors of “information and expertise” can be expected to dismiss his exhortations. The supposed divide between idealism and realism continues to undergird the economic, military, political and environmental status quo, as if the so-called realists have not been, again and again, catastrophically wrong.

James Carroll is Distinguished Writer-in-Residence at New York University and author, most recently, of Christ Actually: Reimagining Faith for the Modern World.