At the time of his speech at Riverside Church, King had come to see war, poverty, and racism as interrelated; taking on one necessarily meant confronting the others. PHOTOGRAPH BY AMERICAN STOCK / GETTY

Fifty years ago, John Lewis, the civil-rights activist and current congressman from Georgia, was living in the Chelsea neighborhood of Manhattan, in a studio on Twenty-first Street. On April 4, 1967, he rode uptown to Riverside Church, on the Upper West Side, to hear Martin Luther King, Jr., deliver a speech about Vietnam. Lewis knew that King would declare his opposition to the war, but the intensity and eloquence of King’s speech, titled “Beyond Vietnam: A Time to Break Silence,” stunned him. What King offered was a wholesale denunciation of American foreign and domestic policy. He had never spoken with such fathoms of unrestraint. For Lewis, the force of the speech eclipsed that of all the others that King gave, including his most famous.

“The March on Washington was a powerful speech,” Lewis said to me recently, over the phone. Lewis was present for that one, too: he spoke on the steps of the Lincoln Memorial minutes before King did. “It was a speech for America, but the speech he delivered in New York, on April 4, 1967, was a speech for all humanity—for the world community.” He added, “I heard him speak so many times. I still think this is probably the best.”

Half a century later, the Riverside speech also seems to carry the greater weight of prophecy. King portrayed the war in Vietnam as an imperial one, prosecuted at the expense of the poor. Vietnam, he said, was “the symptom of a far deeper malady within the American spirit,” and, if left untreated, if the malady continued to fester, “we shall surely be dragged down the long, dark, and shameful corridors of time reserved for those who possess power without compassion, might without morality, and strength without sight.”

“It was far beyond anything I thought he would say,” Clarence Jones, King’s attorney and speechwriter on many occasions, told me. Initially, Jones and other members of King’s inner circle advised him not to give the speech. Any public utterance against Vietnam would threaten his relationship with President Lyndon Johnson, who had helped to advance the cause of civil rights. And King was in a beleaguered spot in 1967. His organization, the Southern Christian Leadership Conference, was broke. Their Chicago campaign of the previous year—an attempt to make inroads in what King called “the teeming ghettos of the North”—had been a failure. More than ever, perhaps, he needed Johnson. So why step into Riverside? Jones and the others wondered. Politically, there was nothing to gain.

But King was not thinking politically, not in that sense. Jones recalled preparing some remarks, only to have King dismiss them on account of their hedging and diplomacy. King called him on the phone, Jones remembered, “and he says, ‘What’s all this on the one hand and on the other hand?’ ” King saw no reason to be circumspect or honor multiple sides. “The Vietnam War is either morally right or morally wrong,” he told Jones. “It’s not on the one hand or on the other hand.”

For assistance, King turned instead to Vincent Harding, a professor of history at Spelman College, in Atlanta. Harding had written a speech for King earlier that year, “The Casualties of the War in Vietnam,” delivered in Los Angeles, in February. Harding knew King’s turn of mind; he had heard King criticize the war in sermons at Ebenezer Church, and he had made his own views known in an open letter to the Southern Christian Leadership Conference in 1965, when he urged the group to adopt a more outspoken stance against American conduct in Vietnam. “I was not putting ideas into Martin’s head, not even words so much into Martin’s mouth,” Harding, who died in 2014, later said. He was, he explained, “doing something for him that he didn’t have time to do for himself.”

In drafting the Riverside address, Harding used many passages from the earlier speech. But he and King also made several changes in structure and emphasis, and a few crucial additions. The most memorable line from Riverside was not spoken in Los Angeles. Listing the reasons why he felt compelled, on April 4th, to protest the war, King recalled visiting cities in the wake of riots and the guilty thoughts that attended his pleas for nonviolence: “And I knew that I could never again raise my voice against the violence of the oppressed in the ghettos without having first spoken clearly to the greatest purveyor of violence in the world today: my own government.”

Part of the Riverside speech is a missive to the American government, and a dire one. King mourned the way that the war in Vietnam had diverted money from Johnson’s Great Society programs, and he noted that the casualty reports showed a disproportionate share of the poor and people of color. In Los Angeles, King had described Vietnam’s move for independence in 1945, and America’s decision to block that bid for self-rule and support France in its war to take control of the peninsula. “We again fell victim to the deadly Western arrogance that has poisoned the international atmosphere for so long,” King said. These paragraphs appear in the Riverside speech as well, but there they become the prelude to a new section, in which King outlines the history of the war as it might be perceived by a peasant in Vietnam:

We have destroyed their two most cherished institutions: the family and the village. We have destroyed their land and their crops. We have cooperated in the crushing of the nation’s only noncommunist political force, the unified Buddhist Church. We have supported the enemies of the peasants of Saigon. We have corrupted their women and children and killed their men.

It was an acute and devastating threnody, which King read in a methodical cadence. He never broke from his script to preach that night—he scaled back his usual range of articulation to sound, Jones said, like “he was speaking a dissertation.” By the time he had gotten through the above section, he had exhausted any hope of finding a credible reason for the U.S. to maintain its involvement in the conflict. And this was nine months before the Tet offensive, a year before the massacre at My Lai. King was not backing into a widely sponsored position.

But ultimately he had his sights beyond the current war. “We as a nation must undergo a radical revolution of values,” he said. “A nation that continues year after year to spend more money on military defense than on programs of social uplift is approaching spiritual death.” King warned of a time of endless war, when the U.S. would be trapped in one overseas entanglement after another while the gap at home between the rich and poor grew ever larger.