On the day I spoke to author Lawrence Roberts about his new book on the 1971 Mayday protests in Washington, news broke that the Defense Department had inquired whether a heat ray might be available to use against Black Lives Matter protesters in Lafayette Square in D.C.

It was easy to imagine the paranoid figure at the center of Roberts’s fascinating history, “Mayday 1971,” desiring just such a weapon to use against his fellow Americans. When 40,000 young people intent on ending the Vietnam War came to Washington 49 years ago, President Richard M. Nixon wondered why the police didn’t force them from the streets with fire hoses.

And when an explosive was found underneath a D.C. bridge, Nixon was disappointed to learn that it was fake — and thus couldn’t have gone off. A shattered bridge would have been good for him.

“The idea that the president of the United States would prefer that a bridge over the Potomac be blown up just so he could use it as a political wedge is pretty startling,” Roberts said.

Roberts’s book is a sweeping account of the protests, told from multiple points of view, from the counterculture figures who planned the protests to the law enforcement and Justice Department leaders who tried to deal with them, from the hippies who took part to the public defenders who had to deal with the legal aftermath.

It’s impossible to read “Mayday 1971” without thinking of our own recent summer of unrest — and the autumn of discontent we face.

“The parallels are certainly really striking,” said Roberts, 68, a former editor at The Washington Post who lives in Garrett Park, Md. “One of the reasons I started to write the book was that even before things got really crazed in the political realm, it looked like American democracy was in trouble. There were so many things going on that seemed to be unraveling. Institutions were on the brink. And the last time we went through a period where it felt like the country was going off the rails was the ’60s and early ’70s.”

It was an era of protest. The right of citizens to register their dissent is “baked into the whole machinery of American democracy,” Roberts said. “And how a sitting government deals with that dissent really tends to expose the best and the worst of the players involved. We’re seeing that again.”

To deal with the events that took place from May 1 to 4, 1971, some 10,000 federal troops were mobilized. Military helicopters landed on the Mall and disgorged soldiers. Police fired tear gas and charged protesters. Among the 12,000 people scooped up by police — still the largest mass-arrest in U.S. history — were countless innocent bystanders.

Despite the size of the crowds and the passion of the protesters, Roberts said there was almost no violence or property damage.

“One of the lessons of Mayday is that massive civil disobedience can be carried out in a completely nonviolent way,” he said.

Another lesson: Getting pulled into the orbit of a toxic administration often does not end well. Many people came to the Nixon White House with a true sense of public service only to find they had to appeal to the president’s darker instincts to curry favor.

“As people go down that road, they lose their values, they give up their own sense of ethics, their yardstick to measure right and wrong,” Roberts said. “They become yes men. That’s the way you survive and advance, through groupthink. You see what the results are.”

The book is not without its grimly comic moments. Washington’s police department was among law enforcement agencies that had placed informants among the protesters. These spies were told that when the hammer came down on the day of the march, the password “bent penny” would let the cops know whose side they were on, sparing them from violence and arrest.

“The authorities had apparently forgotten to clue in the police about the plan,” Roberts writes. One informant took a nightstick to the mouth, the cop who swung it apparently thinking that “Bent penny! Bent penny!” was an insult.

Roberts’s book relies on previously unexamined Nixon White House tapes that show just how involved the president was in planning the response to the protesters. This response, a court would later decide, was unconstitutional.

Roberts said he completed “Mayday 1971” impressed with the resiliency of democracy.

“The authority of the justice system, the ability to self-correct, was really striking and heartening and felt inspiring,” Roberts said. “So I was hoping that the book would provide a lesson in that, that we are able to survive really fractious times and carry on.”

Twitter: @johnkelly

For previous columns, visit washingtonpost.com/john-kelly.