I thought of it as a sin of omission. The nuns thought of it as a subversive act.

It was 1969 when I invited Howard Zinn to speak to an assembly at my small Catholic high school on the non-Trotskyite side of Cambridge. I told the principal he was a government professor at Boston University with a special interest in civil rights and American foreign policy. Not an inaccurate description. Not a comprehensive one, either.

Zinn was then better known for a trip to Hanoi where Ho Chi Minh had turned over to him and the Rev. Daniel Berrigan three US airmen who had been shot down over North Vietnam. I didn’t mention Berrigan to the principal. Him, she’d heard of.

It is hard to say now what I was thinking then. Only, perhaps, that it was time we heard another perspective on the war in Vietnam. The war was closer to us than it was to the well-heeled college students a mile down Mass. Ave., making so much noise about it in Harvard Yard. Our brothers were there. Theirs weren’t.

Mine was on an aircraft carrier in the Gulf of Tonkin. Any attempt at home to question the war was met with the stern rebuttal that dissent gave aid and comfort to the enemy when Danny was in harm’s way. Maybe that is why school seemed a safer place to air the doubts that had even prompted our local congressman, Thomas P. O’Neill Jr., to break with President Lyndon B. Johnson and oppose the war.

Even then, I was a voracious newspaper reader. I had read about Zinn’s trip to Hanoi, about his book Vietnam: The Logic of Withdrawal. I knew he would have a different perspective than Sister Emerita, our American government teacher who embraced the domino theory and whose anti-Communism extended to an insistence that we refer to China as “Red China” unless we were discussing Taiwan.

It was from Sister Emerita that I saw the first signs of trouble when I walked with Professor Zinn into the auditorium. Whether she recognized him from televised clips of his arrests during civil rights and anti-war protests or she suddenly connected his name to his views, I never learned. She did stand from her metal folding chair as I gave him a fuller introduction than I had provided the principal.

As he began to speak, she turned her back on him.

That would have been my sharpest memory of that day had Howard Zinn not demonstrated the calm and grace that characterized the life and work of this self-described “radical professor.” His death last week at 87 brought the day back in bold relief. He accepted her interruptions and her questions. A catcall of “Commie” from a student caught him off guard, diverting him into a brief discussion of Marxism and social justice the likes of which that assembly had never heard.

I would say that Sister Emerita and the dissenters won the day — Zinn did not get to finish his presentation — but for what followed when I walked him to the door. He smiled when the principal said she would be waiting to see me when I returned. “Free speech isn’t free,” Howard Zinn told the quaking high school newspaper editor I was then. “But the courage of our convictions is all we have. Go back in there and take the consequences.”

I stopped quaking and did as I was told.

Eileen McNamara, a former Pulitzer Prize-winning columnist at The Boston Globe, is a professor of the practice of journalism at Brandeis University.