On the morning of March 16, 1968, U.S. Army soldiers entered a Vietnamese hamlet named My Lai 4 on a search-and-destroy mission in a region controlled by Viet Cong forces that the Army referred to as “Pinkville.” The soldiers didn’t encounter any enemy troops. Yet they proceeded to set huts on fire, gang-rape the women, and murder some 500 unarmed civilians including approximately 50 children under the age of four.

On the 50th anniversary of the My Lai massacre, the barbaric act still remains difficult to fathom. The massacre stands among the most infamous of wartime atrocities committed by any U.S. military force.

When news of the massacre finally hit newsstands more than a year and a half after it had occurred, it swiftly became emblematic of the U.S. war effort in Vietnam. Especially in the eyes of the war’s critics, the massacre was proof that America’s moral compass no longer functioned, as well as evidence that the government’s claim of “defending” Southeast Asia from atheistic communist aggression had become a cruel and paradoxical hoax.

The military’s secrecy ultimately compounded the shock of the revelations once they became public. Not only had scores of Army soldiers participated in the wanton murder of defenseless women and children, but the Army’s leadership had seemed to conspire to sweep crimes against humanity under the carpet.

The Tet Offensive that preceded the massacre at My Lai by less than two months led to graphic televised scenes and photographs that gripped the American public day after day. In contrast to the instantaneity of Tet’s news coverage, My Lai triggered a cover up by the Army that served to keep the massacre secret from the American public for a staggering 20 months during an election year. The U.S. military had deceived the public about the course of the war for years, but this was a concerted effort to hide an act of barbarism and turn it into a resounding victory over the Viet Cong.

The atrocity itself was a deeply inhumane act. American soldiers stabbed, clubbed, and carved “C [for Charlie] Company” into the chests of their victims; and herded them into ditches and blew them to bits with grenades. One soldier recalled cutting victims’ throats and chopping off their hands. “A lot of people were doing it and I just followed,” he said. “I lost all sense of direction.”

In his gripping account My Lai: Vietnam, 1968, and the Descent into Darkness, Howard Jones reveals that this collective act of barbarism was far from an aberration by a small group of frightened and confused men, but a predictable result due to the way the war was being waged. The Army had dehumanized the Vietnamese people as “Gooks” and depicted women and children as potentially lethal combatants, while jungle warfare had fostered “an environment in which [U.S. troops] could not be sure who or where the enemy was,” Jones writes.

Members of Charlie Company, which committed the bulk of the atrocities, had seen comrades killed by land mines and sniper fire, even heard one being skinned alive, and in the words of one troop, they had become “leaderless, directionless, armed to the teeth, and making up their own rules…” Their confusion was only bolstered by official military policy. At the time, the combat strategies and tactics in use—including an emphasis on “search-and-destroy” missions and “free-fire zones”—encouraged troops to destroy entire hamlets and villages and defoliate forests. This, in turn, inflated enemy kill rates (the Pentagon’s measure of the war’s progress).

Some men were afraid to speak out about what had happened, and Army officers who heard eyewitness reports of a massacre were quick to discount them. The Army’s Public Information Office issued a press release that informed news coverage, but it was riddled with falsehoods. The Army claimed its troops had killed 128 Viet Cong, even though they had in fact met no resistance and suffered only one self-inflicted wound; Lieutenant Colonel Frank Barker, who helped to oversee the incursion, bragged that “the combat assault went like clockwork,” Jones reports.

Although the public would not find out what happened for nearly two years, word of the atrocity quickly spread among troops in Vietnam. Some American GIs refused to remain silent about the Army’s cover up of the grisly deaths of unarmed women and children at the hands of U.S. soldiers. Helicopter pilot Hugh Thompson, for example, had tried to stop some of the soldiers from massacring civilians during the assault, and he and two others informed commanders about the war crime within hours, to little avail.

Helicopter door gunner, Ronald Ridenhour, who had been told of the My Lai killings by soldiers who had taken part in the slaughter, returned to his home in Phoenix and compiled a dossier of facts about it. On March 18, 1969, almost one year to the day of the massacre, Ridenhour sent a letter to 30 Washington officials detailing the My Lai massacre. Two investigations—one focused on establishing whether a massacre had occurred; the other into a potential cover up by Army brass—were launched.

Soon, freelance reporter Seymour Hersh got a tip that Charlie Company’s Lt. William Calley was being court-martialed on charges that he had killed Vietnamese civilians. Hersh interviewed Calley about his role in the slaughter, but Calley insisted that My Lai had been a fierce firefight with the Viet Cong, not an assault on unarmed villagers. Hersh talked to others who were there, however, and in November 1969 he reported that an unparalleled atrocity had taken place in My Lai in a graphic story that appeared in dozens of newspapers.

The Cleveland Plain Dealer soon published Army photographer Ronald Haeberle’s private photos documenting the slaughter. These first news accounts pricked the nation’s conscience and set off a debate about what had actually happened, what the event said about America’s war effort, and who bore responsibility for the massacre.

More than a dozen military servicemen were eventually charged with crimes, but Calley was the only one who was convicted. In spite of growing opposition to the war, much of the American public remained supportive of its soldiers, and was reluctant to pin the blame on them for simply following the orders of their commanders. This climate made it harder to charge senior military leaders, let alone win convictions in military courtrooms.

Millions of Americans actually considered Calley a “scapegoat” who had been “held responsible for almost a decade of mistakes in Vietnam,” in the words of Alabama congressman Walter Flowers. President Richard Nixon reduced Calley’s sentence to a light punishment—three years of house arrest. The My Lai massacre, the Army’s cover up, and the military courts’ abject failure to bring military leaders and GIs to justice further undermined the faith of some Americans in their political and military institutions.

My Lai didn’t occur in isolation, of course. The U.S. government for years had deceived the public about the war’s progress, and dogged investigative reporting including publication of the Pentagon Papers were beginning to reveal the level of official deception. After the Tet Offensive in early 1968, a majority of the American public came to view the war as a mistake, and the subsequent cover up of My Lai served to deepen people’s despair that the war could ever be won. The massacre also raised big questions about whether the United States was capable of defending freedom, democracy, and human rights in far-flung places.

Rather than liberating concentration camps and promoting universal notions of human dignity, the United States now seemed to some Americans to have been complicit in covering up war crimes. At the same time, Americans who still supported the war in 1969 thought that Calley was just carrying out orders and had become the fall guy for higher-ups looking to take the spotlight off their own Vietnam-era blunders. Finally, My Lai created an unflattering portrait of GIs, and led to the shoddy treatment some returning soldiers received when they rotated back home from Vietnam.

The My Lai massacre and the Army’s cover up are a particularly dark moment in the history of modern America. Had the Army taken reports of atrocities seriously from day one; had it launched an investigation and made the findings public, perhaps the country would have had more faith in its institutions. Although the system ruptured in horrific ways during the assault on My Lai, others might have seen it working by holding those responsible for the crimes accountable. That’s not the way the aftermath transpired, and as a result, the legacy of the massacre remains even more haunting than it might have been.

Matthew Dallek, associate professor at George Washington University’s Graduate School of Political Management, is author, most recently, of Defenseless Under the Night: The Roosevelt Years and the Origins of Homeland Security.