In the spring of 1971, Richard Nixon found himself in a situation not unlike President Trump’s. His approval rating was falling — in Mr. Nixon’s case, to a first-term low — just as an energetic social movement was hitting the streets. Like Mr. Trump, Mr. Nixon was tempted to use military force to counter those dissenters. And like the current president, Mr. Nixon and his aides found a way around the Pentagon’s resistance.

The occasion was the most audacious plan yet by the six-year-old movement against the Vietnam War. A group called the Mayday Tribe organized a traffic blockade of Washington under the slogan “If the government won’t stop the war, we’ll stop the government.”

As the Mayday action unfolded on May 3, twin-engine Chinook helicopters roared down by the Washington Monument, disgorging troops from the 82nd Airborne Division, who trotted off to the Capitol and other hot spots. In all, the administration summoned 10,000 soldiers and Marines, turning “the center of the nation’s capital into an armed camp with thousands of troops lining the bridges and principal streets, helicopters whirring overhead and helmeted police charging crowds of civilians with nightsticks and tear gas,” according to a New York Times report. More than 12,000 people were swept up over three days, the largest mass arrest in U.S. history.

John Dean, the Nixon aide who flipped on his boss in the Watergate scandal, wrote recently in The Times: “Never once did I hear anyone in the Nixon White House or Justice Department suggest using United States military forces, or any federal officers outside the military, to quell civil unrest or disorder. Nor have I found any evidence of such activity after the fact, when digging through the historical record.”

Mr. Dean and I were there on Mayday (he was inside the White House; I was on the streets). He has suggested that the troops were called by city officials, not Mr. Nixon, and in any case weren’t used offensively to quell the blockade. I also dug through the historical record, for a new book on those events, and came to quite a different conclusion. What I found in White House tapes, in minutes of planning meetings and in the papers of Mr. Nixon’s aides, including those of his chief of staff, H.R. Haldeman, and his chief domestic adviser, John Ehrlichman, left no doubt that a half century ago, a president under siege resorted to military force and mass arrests for political gain.

The Mayday protest was the finale of an extraordinary season of dissent. After Mr. Nixon expanded the Vietnam War into Laos, hundreds of thousands of protesters arrived in Washington for a variety of events. Among them were Vietnam veterans, “flower children,” self-styled revolutionaries and pacifists. Veterans hurled medals onto the Capitol’s steps. Quakers held pray-ins. A mass march, almost surely the biggest the city had seen, stretched along the National Mall. Then, on the first weekend in May, more than 40,000 people gathered by the Potomac River for the Mayday action.

The antiwar movement had already helped turn public opinion against Mr. Nixon’s conduct of the war. He was determined to deny activists a victory that could cause further political damage. He blasted them in private with rants like “Little bastards are draft dodgers, country-haters or don’t-cares.”(If Mr. Nixon had access to Twitter, his tweets would have been eerily similar to Mr. Trump’s.) He instructed aides to ensure the blockade would fail and, as one put it, didn’t care if it took 100,000 troops, and if they came up short, “someone will be in big trouble.”