“King in the Wilderness” is a provocative title for a Martin Luther King Jr. documentary, because it creates an image so counterintuitive it’s disarming. In the twelve years he strode across the national stage — from the end of 1955, when the Montgomery Bus Boycott began, through 1968, the year he was assassinated — King was a beacon of transcendent fire and radical moral courage. That was just as true during the last 18 months of his life, the period covered by Peter Kunhardt’s eye-opening, meticulous, and haunting movie. So why, during that time, was King in the wilderness?

Because the world around him had changed. The landmark events led by King during the Civil Rights era (the Bus Boycott, the 1963 “I Have a Dream” speech, the passage of the 1964 Civil Rights Act and the 1965 Voting Rights Act) were all marked, in the eyes of the nation, by a foundational clarity. But the issues that King was focusing on now, like workers’ rights, while every bit as urgent and morally driven, were less iconic. They lacked the stone-tablet majesty of the earlier battles.

King, too, now faced a major dissension within his own ranks. The gospel of non-violence was a stunningly effective strategy that had allowed King, in almost any situation, to occupy the moral high ground. But by the time 1967 rolled around, a new generation of black leaders, inspired by Malcolm X (who was killed by the Nation of Islam in 1965), effectively flipped the meaning of non-violence. To defend yourself if attacked, said Malcolm, was not only justified — it was an essential action of dignity.

To Stokely Carmichael, the future Black Panther who started off, alongside King, as a leader of the Student Non-Violent Coordinating Committee (SNCC), the prospect of turning the other cheek now rubbed him the wrong way. In “King in the Wilderness,” there’s an extraordinary piece of footage in which King and Carmichael are strolling side by side, leading a march in the blazing sun, and a reporter with a microphone toggles between them, asking each of them questions about this very subject, effectively moderating a debate on the issue of non-violence. King is trying to spearhead a protest, but the singularity of his philosophy is fraying around him as he speaks.

There’s another, quite famous piece of footage from this period, shown in the movie, in which something that sounds like a gunshot rings out, and King ducks, his eyes darting nervously. That image has the effect of outing King’s anxiety; it captures the unbearable pressure he was under. “King in the Wilderness” includes a wealth of other material that feels revelatory: home-movie clips of King looking wary and earthbound, that solemn majestic phrasing of his now sounding like a controlled cover for fear, as well as new interviews with multiple members of his inner circle (Andrew Young, Jesse Jackson, Bernard Lafayette). Racially, America had become a cauldron that was boiling over, and King was at the center of it, to the point that his non-violent stance looked more extreme than ever. (It was now seen as a provocation.) What “King in the Wilderness” shows us, through close-up archival footage, is the sweat and dread King lived with every day — and probably did back in the Bus Boycott era, too. It just wasn’t made public.

The movie humanizes King, in occasionally startling ways. There’s a clip of him on “The Merv Griffin Show,” speaking with a preacher’s sly smile about the night-world excitement of New York City, and Andrew Young recalls King’s rather shocking sense of humor during this era: He would improvise fake eulogies for one of his cohorts, sermonizing with that operatic gospel voice but making sure to work in scathing putdowns of whoever he was talking about. (He was also, we’re told, an ace impressionist.) Harry Belafonte speaks with candid eloquence of his deep friendship with King, and there’s a clip of King at home in Atlanta, having dinner with his kids and saying nothing to them because he’s too “hungry” — but what we see at that modest kitchen table is a haunted man, possessed by his mission to the exclusion of all else.

On April 4, 1967, King delivered his seminal speech at New York’s Riverside Church condemning the war in Vietnam. His decision to come out publicly against the war, and to attack his Civil Rights ally President Johnson over it, drew the ire of most of the nation: his supporters in the Democratic Party (who, at that point, chose to stand by their president), but also, strikingly, many of his brothers in the Civil Rights struggle, who felt that he was diluting their message. But King couldn’t not speak out. He saw the corruption of the war as inextricable from the United States’ systemic racism. He lost political points, but expanded his breadth as a leader. The fact that so many, including liberals, thought he was “wrong” is the proof, in hindsight, of how right he was.

“King in the Wilderness” raises the question: Was the sunset of the King era on the horizon even before King was killed? The answer is no. Yet there would now be competing visions, a heightened agitation, and multiplying enemies, as the passion for equality spread like wildfire (literally, in the burning inner cities). King himself was now everywhere: north and south, fighting for housing rights in a city like Chicago, where the seething, swastika-wielding white supremacists who gathered to protest against him numbered 10,000 strong. Before, King had fought to integrate buses, lunch counters, and schools. Now he was fighting to integrate neighborhoods. Guess which action provoked more white rage. His team tried to get him to take a sabbatical from protest by becoming the interim pastor at Riverside Church (a post he was offered). He hadn’t had a break in 11 years, but he didn’t consider it. For King, the struggle was all.

He lived, throughout this period, with a sense of the end — not in some eerie-macabre-mystical way, but in his openly stated notion that he didn’t expect to live past 50. The night before he was killed, though, there’s a telling story. He was already in bed, in his pajamas, at the Lorraine Motel when he was asked by his team to give a quick sermon to the Mason Temple. He has no desire go, but got dressed, came over, and literally tossed off the “I’ve been to the mountaintop” speech (“Mine eyes have seen the glory!”). Read into that what you will. “King in the Wilderness” is a searing film because it takes Martin Luther King Jr. down from the mountaintop. You glimpse the real glory of who he was: not a walking monument but a human being with fear, humor, guts, and (amazing) grace under pressure.