Protesters shut down D.C. traffic before. It helped end the Vietnam War — and reshaped American activism.

Demonstrators — hailing from a wide variety of social justice causes — armed themselves with anything they could find: trash, tree limbs, bottles, bricks, tires. They used the materials to form barricades across heavily trafficked sites in the District, stared into the faces of waiting police officers and prepared for a day of conflict.

But this wasn’t the September 2019 Shut Down D.C. protest that interrupted the Washington commute to draw attention to climate change. This was May 1971, the original D.C. traffic blockade launched to protest the Vietnam War — what longtime organizer and historian L.A. Kauffman calls “the most influential protest you’ve never heard of.”

″It contributed to helping end the Vietnam War and pioneered a new model of organizing that would shape movement after movement in the decades to come,” Kauffman said in a phone interview with The Washington Post. “May Day 71 was a crucial turning point in our history.”

Government records show that officials of President Richard Nixon’s administration were so spooked by the massive action that it hastened their withdrawal from Vietnam, Kauffman said. And in the years following May Day, activists across the country adopted the demonstration’s guiding principles, Kauffman said: that movements should be decentralized; should revolve around small, autonomous groups; and should involve participants hailing from all kinds of ideologies.

The protest also featured another notable American landmark: Thanks to what Kauffman calls a “massive overreaction” from the Nixon administration, more than 7,000 protesters were arrested in a day. It was the largest mass arrest in U.S. history, according to Kauffman.

In a text, 2019 Shut Down D.C. organizer Liz Butler confirmed that May Day 1971 served as inspiration for her coalition’s modern version. Thai Jones, a professor at the University of Columbia who studies radical social movements, told The Post it makes historical sense that climate activists today would adopt the tactics of anti-Vietnam War protesters in the 1960s and 1970s.