Zachary Jonathan Jacobson is a Cold War historian. He received his PhD from Northwestern University.

A silent vigil on the front steps of the U.S. Capitol on Oct. 15, 1969, the day of the widespread Moratorium protests against the Vietnam War. (AP)

On Oct. 15, 1969, in 10,000 high schools across the United States, students skipped classes to demand a halt to the Vietnam War. The mass demonstration, dubbed the Moratorium, was “a march against death.” Protesting a war that seemed all but lost, yet still cost more than $2 billion each month, activists called for a nationwide general strike to stop “business as usual.”

Streets filled with rallies and prayer vigils. Church bells tolled and workers struck. Municipal buildings were draped in black crepe, and American flags flew at half-staff. The best estimate was that 2 million Americans demonstrated that day. Life magazine judged the protest “the largest expression of public dissent ever seen in this country.”

The memory of the Moratorium — its breadth and its limits — strikingly echoes the strident efforts of high school protesters today. The survivors of the Feb. 14 massacre at Stoneman Douglas High School in Parkland, Fla., have taken the vanguard to insist on not just “common sense” but thoughtful and significant gun reform. And like antiwar dissenters’ general demands for peace, the  #NeverAgain movement has called more often for systemic change rather than a specific plank of reforms. The Moratorium experience reveals that beginning a process of reform takes time, but persistence may eventually pay off — both in ways seen at the time and ones not visible for years to come.

The key to the Moratorium was its multiplicity and creative force. The Vietnam Moratorium Committee was headed mostly by young veterans of Eugene McCarthy’s 1968 campaign for the presidency. They aimed to channel the nation’s discontent surrounding the war into mass protest, following the example set by civil rights leaders. Their message was simple: End the war.

These members of the “Clean for Gene” McCarthy crowd wanted to re-brand the antiwar effort as a peaceful cause and divorce their movement from the rioting at the Democratic National Convention the previous year and the growing strength of the radical student faction known as the Weathermen.

In short, the Moratorium committee was determined to stay mainstream and to work within the system for political change. By collecting endorsements from groups big and small — from the United Auto Workers to the National Council of Churches, and even corporations such as Midas Inc. — the organizers brought women and men of all ages and from all walks of life into the student movement. Their goal: to win the rhetorical battle and transform public perceptions of the Vietnam War from a “noble cause” to an “immoral war.”

On the day of the demonstration, protesters wore black armbands to mourn the war’s dead. And just as “We Shall Overcome” had been the hymn of the civil rights movement, they sang John Lennon’s new hit, “Give Peace a Chance,” as the day’s anthem.

A hundred thousand gathered in the Boston Common to hear Sen. George McGovern (D-S.D.) speak as a skywriter sketched out the peace sign above them. Another 100,000 converged on Bryant Park in New York, where New York Republican Sens. Charles Goodell and Jacob Javits and Mayor John Lindsay participated. In the nation’s capital, Coretta Scott King greeted 50,000 protesters in front of the Washington Monument as hundreds of thousands more converged to fill the Mall to capacity. The Rev. Ralph David Abernathy, the successor to the Rev. Martin Luther King Jr. as president of the Southern Christian Leadership Conference, led a march in Los Angeles.

And the Moratorium did not stop at the edge of the metropolises. In East Meadow, Long Island, the longtime ambassador and statesman W. Averell Harriman spoke to a crowd of 15,000. Six inches of snow could not deter 3,000 in Denver. The tally was 11,000 in rural Iowa. Nuclear scientists at the Brookhaven National Laboratory halted their atomic research. And across the pond, in London’s Grosvenor Park, just steps from the U.S. Embassy, Bill Clinton, then a Rhodes Scholar, gathered hundreds to dissent.

The organizers had labored to shore up political support to demonstrate the breadth of their movement. The evening before the Moratorium, 23 members of Congress convened under the Capitol dome for a night’s worth of speeches against the war — the longest stretch of antiwar congressional speechmaking to that point.

There was a mighty backlash from Vietnam War supporters, and a lot of name-calling. The fierce-tongued Vice President Spiro Agnew taunted the demonstrators as an “effete corps of impudent snobs.” He doubted their purpose was anything more than “a spirit of national masochism.” And he credited their cause to “the young … overwhelm[ing] themselves with drugs and artificial stimulants.”

Sen. Barry Goldwater (R-Ariz.) denounced the Moratorium for being anti-American and “playing into the hands of people whose business it is to kill American fighting men.” In defiance, 15 Republican congressmen assembled to demand an intensification of the war effort, and conservative counterprotesters marched with signs that read “America, Love It Or Leave IT.”

The Moratorium did not immediately stop the conflict, and data simply does not exist to indicate whether the protest changed hearts or minds. The Moratorium organizers clashed among themselves over money and dates and messaging for future demonstrations. Exhaustion and frustration led to more infighting. The coalition between anti-Vietnam War liberals and radical (at points militant) pacifists dissolved. A withdrawal from Vietnam was not negotiated for three more years; a full withdrawal not for another six.

But while President Richard Nixon refused to comment publicly on the demonstrations, letting “it be known that he was watching sports on TV in the White House” that day, we now know that the president was privately spooked by the Moratorium and its reprise, dubbed “the Mobilization,” a month later in Washington. These protests hamstrung his ability to negotiate with the North Vietnamese, and maybe most importantly forced Nixon to cancel plans to expand the war with an offensive of aerial bombing, harbor mining, even an invasion of North Vietnam. As Henry Kissinger, years later, would remark, “we finally rejected the military option because we did not think we could sustain public support.”

The 2 million-man march demonstrated the sheer numbers in dissent. The unity of action firmly established that the antiwar movement had a constituency that could be mobilized.

And much like today’s swath of protests — Black Lives Matter, Women’s March, #MeToo, gun control — the antiwar activists spanned and overlapped different social movements. Their ranks included moderates and radicals, labor unionists, civil rights groups, women’s rights groups, free-speech activists, clergy, socialists, communists, pacifists, militants and counterculture hippies. The great achievement of the Moratorium was to bring a large swath of Americans together in dissent, establishing that their antiwar cause was not fueled by a dangerous fringe that Nixon could, with a few dirty tricks, stamp out or discredit.

The march also provides another lesson that has been learned by activists for causes as disparate as civil rights, the fight against AIDS and Mothers Against Drunk Driving: Great change is not won in a day’s showing. Even as great as was the 2 million-man Moratorium of Oct. 15, 1969, it took years more to accomplish its goal. For it takes a generation.