By the time he took the stage on August 18, 1969, Jimi Hendrix had enjoyed more success than a kid who grew up in Seattle’s Central District in the 1950s could have reasonably dreamed—adoring fans in the UK and U.S., chart-topping singles, talk of his unbridled electric guitar style transforming the medium. Now, at 26, he was headlining Woodstock in upstate New York, the most important music celebration of a generation.

That morning Hendrix and his band hammered through the hits (“Spanish Castle Magic,” “Foxey Lady,” “Fire”). Then, near the tail end of the set, they paused. From Hendrix’s guitar came a handful of eerily familiar notes—the beginning of his interpretation of “The Star-Spangled Banner.”

The next few minutes would go down as the most emblematic performance of the 1960s, not just by Hendrix (who would die of an overdose 13 months later) but by anyone: The national anthem rendered as a serpentine, probing bolt of feedback.

“It’s the single most powerful performance of the anthem in the history of the song,” says Mark Clague, an associate musicology professor at the University of Michigan who heads the Star Spangled Music Foundation, dedicated to celebrating and promoting the anthem. “In some ways it’s a kind of musical depiction of democracy…in the sense that it’s a cacophony of disagreement.”

It wasn’t the first time the guitarist played the song, but the Woodstock iteration stands out for three reasons: the quality of the performance, the popular 1970 documentary about the festival—for which the “Banner” is a soaring denouement—and the context. It was the end of a tumultuous decade. The Vietnam War was raging. JFK and MLK had been shot dead. So had student protestors. And nearly 500,000 people, most of them boomers, then in their late teens and early 20s, had gathered for a rock music exposition that soon doubled as an interrogation of where the country was headed.

That’s one reason Clague is intrigued by the song’s punctuation. After all, the first verse of Francis Scott Key's 1814 lyrics, the basis for the anthem, ends with a question mark. Are we going to live up to our ideals? Can we make this country “the land of the free”? But we sing it now—as most people did in the ’60s—as if it’s an exclamation mark. The Hendrix version, says Clague, “makes you think about what it means to be American, about who gets to be American, who gets to decide what an American does.”

In other words, Hendrix reintroduced the question mark. Ask someone who was there, and they’ll tell you that interrogation resonated in the moment.

On that Monday morning Marc Leepson, 23, recently of the U.S. Army’s 527th Personnel Service Company in Vietnam, watches the day break on a world of mud. Mud seeping up tent walls. Mud under fingernails and clinging to everyone’s bell-bottoms. Leepson spent last night like he has the previous two: partying on little to no sleep with hundreds of thousands of others, listening in the rain to the greatest collection of live rock music ever assembled—Grateful Dead, Creedence Clearwater Revival, Janis Joplin, The Who, Jefferson Airplane.

Now it’s 9am, the beginning of the work week, and Woodstock’s crowd of half a million has shrunk to less than 40,000 as attendees roll out of the muck and back to their real lives. Not Leepson. Discharged from the military a month earlier, he traipses through the sludge for one more march.

Jimi Hendrix, the guitar virtuoso from Seattle, mounts the main stage for the festival’s final performance. Leepson and his friends groove to the lyrics they know so well, anticipating “Purple Haze,” which back in Vietnam was practically the theme song of Leepson’s army unit. Then something changes. The sound issuing from the stage becomes stranger.

For the next four minutes and three seconds Hendrix’s guitar is an instrument possessed, mimicking planes in the sky, bombs dropping and exploding, sirens, and people screaming—the sounds of war and a country unraveling. “We were slack-jawed,” Leepson will recall 50 years later. “Our mouths were hanging open.”

Ahead of Leepson lay a successful career as a historian writing well-received books on the Vietnam War and the American flag. He would also author a biography of Francis Scott Key, the amateur poet who penned the words of the anthem 205 years ago. 

What would Key think of Jimi Hendrix performing his poem?

“This guy was extremely conservative,” Leepson says of the nineteenth-century poet. “In the meaning of conservative as someone who is very, very traditional and doesn’t like change.” He wouldn’t have known what to make of Hendrix—the way he sounded, the way he looked, or that there was a black man leading an American revolution.

A few songs after the “Banner,” Woodstock came to a close. Leepson and the remaining crowd snaked out of the muddy field and fanned out across the country, that big question mark following them to the end of the century and into the next.