In April 1970, Alan Canfora, a junior at Kent State University in Ohio, was outraged when a friend was killed in the Vietnam War. He was infuriated all the more when President Richard M. Nixon announced an expansion of the war into Cambodia.

Nixon’s action set off a wave of antiwar demonstrations across the country, including at Kent State, where the Ohio National Guard was called in to respond to destruction and to be a presence at a major demonstration planned for May 4.

The day began with brief skirmishing; students threw rocks at the Guard and the Guard fired tear gas at the students, whose numbers would swell from the hundreds to the thousands.

At one point, some soldiers knelt and aimed their weapons at the students, in an apparent bluff that they were going to fire. Mr. Canfora then walked out toward the soldiers by himself, waving a black flag.

The members of the Guard stood up and moved to a hilltop. Then, suddenly, 28 of them turned in unison and opened fire on the unarmed students.

They fired up to 67 shots in 13 seconds, killing four students — Allison Krause, Jeffrey Miller, Sandra Scheuer and William Schroeder — and wounding nine others. Mr. Canfora was among the wounded, shot through his right wrist as he darted behind a tree.

A photograph of Mr. Canfora waving his flag was part of a spread in Life magazine that became emblematic of the events of that day, one of the epic confrontations of his generation, when warriors in tactical gear gunned down students on an American college campus. Millions of students across the country went on strike, forcing hundreds of colleges and universities to close and bringing the war home in a visceral way that captured the political and cultural upheaval of the era.

Mr. Canfora went on to become a walking encyclopedia on all aspects of “Kent State,” the university’s name becoming synonymous with the shootings and with state-sanctioned violence — so much so that in 1986 the university tried to rebrand itself as “Kent.” Mr. Canfora spent the rest of his life making sure the university would never erase May 4 from its history.