In 1967, Mr. Duncan joined the Rev. William Sloane Coffin Jr., Dr. Spock, David Dellinger and other leaders of the National Mobilization Committee to End the War in Vietnam, to plan and carry out huge protest rallies in New York, Washington, San Francisco and dozens of other cities, at which draft cards and American flags were burned and protesters clashed with the police.

Mr. Duncan also testified that year at an unofficial “war crimes tribunal” organized by the philosopher Bertrand Russell in Denmark, and at a South Carolina court-martial, where he spoke in defense of Capt. Howard R. Levy, a Green Beret who had also turned against the war. Captain Levy was convicted of disobeying orders and attempting to incite disloyalty, and eventually served 26 months in prison.

In 1968 Mr. Duncan, then the military editor of Ramparts, helped Mr. Dellinger, Mr. Gregory, Rennie Davis and other antiwar leaders plan protests at the Democratic National Convention in Chicago. The rallies and marches drew tens of thousands of protesters and led to hundreds of arrests and injuries, and to a circuslike trial of eight protest leaders on inciting-to-riot charges.

In his Ramparts articles and a memoir, “The New Legions” (1967), Mr. Duncan detailed a military career that began in December 1954, when he was drafted by the Army in Rochester, a 24-year-old American who had been born in Canada and raised by a stepfather of Hungarian origin.

“I was a militant anti-Communist,” Mr. Duncan wrote in Ramparts. “Like most Americans, I couldn’t conceive of anybody choosing communism over democracy. The depths of my aversion to this ideology was, I suppose, due in part to my being Roman Catholic, and in part to the stories in the news media about communism. My stepfather was born in Budapest, Hungary. Although he had come to the United States as a young man, most of his family stayed in Europe.”

Mr. Duncan was serving with an artillery unit in Germany in 1956 when the Soviet Union crushed the Hungarian revolt. He recalled being frustrated and angry when the United States did not intervene. He joined the Special Forces in 1961. “I was so impressed with their dedication and élan,” he said. “Their anti-Communism bordered on fanaticism.”