It turns out there won't be an event at Kent State University this year commemorating the killing of four university students there during a campus protest against the Vietnam war on 4 May 1970. The shootings, carried out by Ohio national guardsmen 41 years ago this Wednesday, shocked the national conscience – and probably helped force the Nixon administration to wind down the Vietnam war more quickly than it intended.

Even today, iconic images from the shooting – most notably, the anguished face of 14-year-old runaway Mary Ann Vecchio, as she leans over the body of one of the dead students, which won a Pulitzer Prize that year – remain disturbingly resonant. They remind us of a time when America was bitterly divided along racial and regional lines, and experiencing violent conflict almost daily. Mere words – and non-violent protest – could get you killed.

The assassination in 1968 of Martin Luther King Jr and Robert Kennedy had already disabused the nation of the idea that only poor, disenfranchised blacks in the south could be victims of violent prejudice and hatred. But now, the deaths of Jeffrey Miller, William Knox Schroeder, Allison Krause and Sandra Scheuer – barely 20, and all good students with promising careers ahead of them, suggested that the Vietnam war had finally "come home". After years of bombing and burning South-east Asian villages in search of an elusive and ill-defined "enemy", the nation's imperial war machine had finally decided to turn its rifles and bayonets on its own privileged children.

Absurdly, perhaps, two of the dead and several of the nine wounded when a small troop of guardsmen suddenly, and without warning, fired 67 shots in the direction of dispersing demonstrators weren't even there to protest the war. Scheuer was crossing the campus parking lot en route to her next class. And Schroeder, a campus basketball star, was actually a member of the campus ROTC recruitment centre, which protesting students had burned to the ground just three days earlier. He'd simply stopped by the protest, and remained on the periphery, to see what all the fuss was about.

The proximate cause of the Kent State protest was the Nixon administration's announcement on 30 April 1970 that it was authorising a military invasion of Cambodia to attack and destroy North Vietnamese communist "sanctuaries". Nixon had been elected largely on his promise to bring the Vietnam war to a close quickly, in part by "Vietnamising" the conflict, which meant bringing US troops home. But he'd never revealed his secret plans to escalate the bombing of North Vietnam, or to draw neighbouring countries into the conflict. Democrats, of course, had long considered Nixon an anti-communist hatchet man, beginning with his political smear campaign against a Democratic opponent, Helen Gahagan Douglas, that launched his political career, and helped earn him the nickname "Tricky Dick".

Nixon also harboured a deep and paranoid animus towards student protesters, whom he frequently called "bums" when he wasn't labelling them "communists". That sentiment spread to other political figures, including Ohio Governor James Rhodes, who was widely viewed as a possible vice-presidential running mate for Nixon in 1972. Four days before the Kent State shootings, Nixon had concluded his speech announcing the Cambodian invasion warning that "we live in a time of anarchy, abroad and at home", and that he would not tolerate attacks on the "great institutions which have been created by free civilizations in the last 500 years", especially, he noted, universities.

And on Sunday 3 May, after a third day of protest at Kent State in which guardsmen had already bayoneted several students, Rhodes denounced the protesters as "unAmerican" and said "they're worse than the brown shirts and the communist element and also the night riders and the vigilantes. They're the worst type of people that we harbour in America". He promised that the national guard would "restore order". And with grisly results, that's what they did.

Public reaction to the killings was swift. Students at some 900 universities and colleges launched a fresh wave of protests, resulting in the first successful student strike in US history. Kent State itself remained closed for six weeks. But the country at large remained as divided over Kent State as it was over the war. A Gallup poll found that 58% of Americans blamed the students for what happened; only 11% blamed the guardsmen, and nearly a third, 31%, remained "undecided". For some, the burning down of the ROTC centre, and the throwing of rocks by students constituted a provocation, and guardsmen interviewed later said they'd genuinely feared for their lives. The campus administration had also banned further campus protests after 3 May, but the students persisted in rallying anyway. And they repeatedly refused to disperse after the guard fired tear gas and tried to clear the area with a minimum of force.

But could anything, in fact, justify the guardsmen, without warning, or apparent provocation, firing on unarmed students?

Even vice-president Spiro Agnew, a former prosecutor, and no friend of the protesters, stunned conservatives when he admitted that while not premeditated, the guardsmen's actions constituted "murder". Interestingly, though, no court ever found the guardsmen, or their superiors, legally culpable for their actions. Most civil lawsuits were also dismissed. Allison Krause's parents, who sued the state of Ohio, eventually received a token "apology", and $15,000 in cash compensation.

The Kent State administration officially commemorated the killings for five years, then withdrew its support, leaving it to grieving families and supporters to sponsor the annual event. But last year, on the 40th anniversary, the campus administration, responding to continued protest, finally agreed to spend the entire day educating the campus about the events and their implications. To some, Kent State may seem like a symbol from an era that has long passed. But thanks to such commemorative events – and the monuments erected in honour of the dead – it's also a testament to the bitter social and political divisions that continue to simmer in America, and a reminder of the dangers to civil liberties and social peace that can arise when the nation goes to war, and sends thousands of its own youth to die on foreign battlefields for seemingly no good purpose.

No campus today is erupting over recent American interventions in Iraq, Afghanistan or Libya. But the memory of campuses transformed into war zones is still fresh in the minds of US military planners, as they seek to fashion limited engagements relying on a strictly volunteer army. With even the Tea Party movement now calling for an end to wars that needlessly drain the nation's treasury, were Obama or another US president to reinstitute a military draft to put unwilling Americans, especially college students, on the ground to fight and die, is there any doubt that a new season of bitter protest could erupt, once again?