Mr. Kleindienst overrode their concerns with an opinion from the Justice Department’s legal counsel, William Rehnquist, who had been his protégé in their home state, Arizona. Mr. Rehnquist said the act didn’t apply; the president had “inherent constitutional authority” to use troops “to protect the functioning of the government.” (Mr. Rehnquist would be named to the Supreme Court by Mr. Nixon later that year and elevated to chief justice under President Ronald Reagan.)
Mr. Kleindienst faced another obstacle. David Packard, the deputy secretary of defense, pointed out the procedures a president should follow, under the Insurrection Act, in calling forth the military: a formal order that demonstrators disperse and, if they don’t, an executive order to send in troops. Mr. Nixon’s predecessor, Lyndon Johnson, had done this during the riots in Washington in 1968 after the assassination of Martin Luther King Jr. The White House, however, wanted to keep its involvement under wraps. According to Mr. Haldeman’s diary, Mr. Nixon let Mr. Packard know he wanted troops sent without any public presidential action. The White House spread the fake news that city officials had requested the military help.
In contrast, Mr. Trump has been open about his desire to send troops to “dominate” streets in cities with Black Lives Matter protests. After Defense Secretary Mark Esper and the chairman of the Joint Chiefs of Staff, Gen. Mark Milley, stood in the way of using active-duty military, the president dispatched forces from agencies including Customs and Border Protection. In June, those agents cleared peaceful demonstrators from Lafayette Square outside the White House for the president’s now-famous photo op in front of a church. In Portland, Ore., they used tear gas and other riot tools to disperse largely peaceful protesters outside the federal courthouse.
During the 1971 Mayday action, as 12,000 people tried to snarl rush-hour traffic with nonviolent civil disobedience, a majority of the regular troops fended off protesters at bridges and federal buildings, or guarded large groups of detained protesters. Most soldiers didn’t confront demonstrators directly, but their presence and hardware bolstered the authoritarian tactics and escalated tensions. A police dragnet swept up 7,000 people that Monday, including many young people just walking on the streets wearing hippie-style clothing, and took in more than 5,000 other demonstrators over the next two days. My research confirmed that Mr. Nixon gave the order to make the mass arrests. He made it clear later to a group of conservative members of Congress: “The point is, I had the responsibility,” he told them. “I approved this plan.”
As criticism mounted that the dragnet was unconstitutional (courts ultimately agreed, awarding detainees millions in damages), Mr. Nixon’s involvement was suspected. The White House denied it. Aides instructed the police chief, Mr. Wilson, to take the heat. “I wish to emphasize the fact that I made all tactical decisions relating to the recent disorders,” he said in a public statement. “I took these steps because I felt they were necessary to protect the safety of law-abiding citizens and to maintain order in the city.” The tapes show Mr. Nixon’s men were delighted.
“Wilson went to the mat today,” Mr. Ehrlichman confirmed to Mr. Nixon. “Good for him!” the president said. Mr. Ehrlichman added, “We programmed him to do this this morning, and he did better than you could possibly have programmed.” He went on: “He has never let us down yet.”
No military leader expressed second thoughts in the weeks after Mayday.
But in June, after General Milley accompanied Mr. Trump to Lafayette Square wearing combat fatigues as protesters were dispersed by federal agents and police, he said he regretted taking part.