On the night of Jan. 31, 1968, as tens of thousands of Viet Cong guerrillas attacked the major cities of South Vietnam, in violation of a Lunar New Year truce, Richard Nixon was flying secretly to Boston. At 29, and Nixon’s longest-serving aide, I was with him. Advance man Nick Ruwe met us at Logan Airport and drove us to a motel in Nashua, N.H., where Nixon had been preregistered as “Benjamin Chapman.” The next day, only hours before the deadline, Nixon filed in Concord to enter the state’s Republican primary, just six weeks away.

On Feb. 2, the New York Times story “Nixon Announces for Presidency” was dwarfed by a giant headline: “Street Clashes Go On in Vietnam; Foe Still Holds Parts of Cities; Johnson Pledges Never to Yield.” Dominating the page was the photograph of a captured Viet Cong, hands tied, being executed on a Saigon street by South Vietnam’s national police chief, firing a bullet into his head from inches away. Eddie Adams’s photo would win the Pulitzer Prize.

America’s most divisive year since the Civil War had begun.

The author and candidate Nixon on a plane during the campaign in September 1968. Photo: Courtesy Nixon Presidential Library

Nixon’s lone opponent for the Republican nomination was George Romney, three-term governor of Michigan and a legend at American Motors, where he had promoted the Nash Rambler. Romney had led in the polls in December 1966 and seemed the clear favorite, but by now he was not.

After campaigning in 35 states in 1966, leading the GOP to its greatest off-year victory in congressional races since 1946, Nixon had declared a moratorium on politics and dropped out of sight. Is it wise, I asked him, to cede Romney such a tremendous head start? Sensing what the press would do to Romney, Nixon told me, “Let ’em chew on him for a little while.”

Nixon’s instincts proved right. Romney was unprepared. On pre-campaign swings in 1967 he bickered with the press, and that August he made a fatal blunder. Explaining on a TV show why he was changing his position on the war, Romney said that on a previous visit to Vietnam, “I just had the greatest brainwashing anybody can get” from U.S. generals and diplomats.

The ridicule and mockery were ceaseless and universal. Sen. Eugene McCarthy said that, in Romney’s case, a full brainwashing was unneeded, as “a light rinse would have sufficed.” Romney plummeted in the polls, never to recover.

As Romney spun his wheels in New Hampshire, Nixon ignored his calls to debate, declining even to mention his name. Our polls showed us heading for a 5-1 landslide that would erase the “loser” image that had clung to Nixon since his loss to JFK in 1960 and his defeat in the California governor’s race in 1962.

President Lyndon Johnson announced on March 31, 1968 that he would not seek reelection; above, he works on the speech the day before. Photo: Bob Daugherty/Associated Press

With humiliation ahead, Romney abruptly ended his candidacy on Feb. 28, 1968, robbing Nixon of his triumph. What historians call “crazy March” now began. In the Democratic primary in New Hampshire, Sen. McCarthy, running an antiwar protest campaign, got 42% of the vote. Lyndon Johnson won with 49%, though his name was not on the ballot. Inexplicably, the president of the United States had run as a write-in candidate.

Half the McCarthy voters were later identified as pro-war but fed up with LBJ’s indecisive leadership. In January, North Korean commandos had assaulted the Blue House in Seoul and come close to assassinating President Park Chung-hee, and the U.S. spy ship Pueblo had been hijacked and its crew taken hostage by North Korean gunboats. Johnson had done nothing.

The press read into the McCarthy vote a repudiation of the war, and Johnson was now wounded. On March 16, Sen. Robert Kennedy leapt into the race. Speaking a week later in Los Angeles, he stuck the knife deep into his old antagonist, accusing President Johnson of “calling upon the darker impulses of the American spirit.”

On March 21, Gov. Nelson Rockefeller of New York stunned the political world by declaring that he would not challenge Nixon. The anticipated battle inside the Republican Party seemed suddenly settled, just as a three-sided war broke out inside the Democratic Party. Alabama’s Gov. George Wallace had announced he would run as a third-party candidate in the fall, while Kennedy and McCarthy battled for the nomination as they assaulted their own president.

The Tet Offensive was seen as a major American setback in 1968 but the Viet Cong lost huge numbers of troops; above, a Viet Cong soldier awaits interrogation following capture. Photo: Corbis/Getty Images

The Tet Offensive proved a strategic disaster for the Viet Cong, who suffered tens of thousands of dead. But U.S. media portrayed Tet as an American defeat. On “The CBS Evening News,” Walter Cronkite declared Vietnam a “stalemate.”

Nixon moved to update his position. As his writers Ray Price, Dick Whalen and I argued in front of him at his Fifth Avenue apartment in New York on March 30, we got a call from our media folks: LBJ had asked to speak in prime time that Sunday night. Nixon canceled his prepared speech and, leaving for a Wisconsin event, told me to be at the private terminal at La Guardia Sunday to brief him on LBJ’s address to the nation.

As Johnson was announcing that he would not run, Nixon’s private jet was landing. I reached the airplane door ahead of the press and told him what LBJ had said. Nixon stepped out into the cameras to declare 1968 “the year of the dropout.”

Four days later, the nation was stunned again. Martin Luther King, Jr., in Memphis to support a strike by garbage workers, had been assassinated on a motel balcony. A hundred U.S. cities exploded in rioting, looting and arson. The National Guard was out everywhere. The weeklong rampage caused a backlash across Middle America, and Wallace’s poll numbers vaulted. Support for Nixon, who went to Atlanta for King’s funeral, sank.

Violence broke across American cities after the April 4, 1968 assassination of Martin Luther King Jr. Above, soldiers stand guard in front of a supermarket on Chicago’s South Side three days later. Photo: Associated Press

As the race riots burned out, the worst campus riot of the decade erupted. At my alma mater, Columbia University, student radicals occupied Low Library and Hamilton Hall. They ransacked professors’ offices and took a dean hostage. After a week, the NYPD, with clubs and sweeping arrests, recaptured the university. Nixon declared the uprising “the first major skirmish in a revolutionary struggle to seize the universities of this country and transform them into sanctuaries for radicals and vehicles for revolutionary political and social goals.”

Rockefeller denounced Nixon, reversed himself and entered the race. But polls showed that America’s patience with radicalism was exhausted. The country was with the cops wielding the clubs. Nixon had captured the law-and-order issue. When the Kerner Commission, set up to study the causes of the weeklong Newark and Detroit riots in the “long hot summer” of 1967, blamed “white racism,” Nixon dismissed the report by saying it blamed everyone for the riots but the rioters themselves.

As the Democratic showdown approached in the Oregon primary, the media zeroed in on the revelation that, as attorney general, Kennedy had authorized J. Edgar Hoover to wiretap the now-martyred Martin Luther King Jr. The explosive charge led to Kennedy’s defeat by McCarthy on May 28.

Senator Robert F. Kennedy’s assassination on June 5 further traumatized the country. Above, he campaigns in Portland, Ore., before the May 28 Oregon primary. Photo: Bettmann Archive/Getty Images

I was at Portland’s Benson Hotel that night with Nixon, who had won 70% of the primary vote, crushing both Rockefeller and Reagan. Later in the evening, I was standing in front of the hotel when Bobby Kennedy arrived to concede defeat in the first loss by a Kennedy since JFK entered politics in 1946. Though Bobby had a reputation for being ruthless, he could not have been more gracious in conceding defeat that night.

A week later, I was awakened at 3 a.m. by Jeff Bell, a young aide at Nixon’s campaign office. Bobby had been shot in a Los Angeles hotel kitchen after winning the California primary. Vice President Hubert Humphrey, the favorite after LBJ stood down, was now assured of the nomination.

The surging antiwar movement was demoralized, bitter and angry. Humphrey was seen as a Johnson lackey who would continue the war. Then, just days after Bobby was buried beside JFK at Arlington, Earl Warren resigned as chief justice, and LBJ named his old crony Justice Abe Fortas to replace him. All three wanted to prevent a President Nixon from naming the next chief justice. Senate Republicans aborted the insiders’ deal and rejected Fortas. The Supreme Court wars that would endure into the 21st century had begun.

The Democratic Convention in Chicago was marked by chaos inside on Aug. 28, 1968, as delegates were fractured over the candidates and the party platform.... Photo: Associated Press

One week before the Democratic convention in Chicago, the Soviet Union sent hundreds of Warsaw Pact tanks and 250,000 troops into Czechoslovakia to crush the Prague Spring. As with the seizure of the Pueblo, President Johnson, with a half million U.S. troops now in Vietnam, did nothing.

The stage was set for an explosive Democratic convention in Chicago. I asked Nixon to send me. He agreed. Our listening post was on the 19th floor of the “Comrade Hilton.” I was alone in the suite one night when Norman Mailer walked in with the light-heavyweight champion Jose Torres. As we talked, a commotion erupted outside. A phalanx of cops had marched up Balbo Drive to Michigan Avenue and halted. Suddenly, the cops took off into Grant Park, clubbing the radicals and dragging them to patrol wagons. Mailer and I saw it all from our 19th-floor window. On and on it went, as Torres cursed the cops and I stayed mute. I had been down there at night among the protesters, who were as ugly a crowd as I had seen in the Vietnam era.

When Humphrey left Chicago, the Democratic coalition that had given LBJ a historic landslide in 1964 was shattered. Wallace seemed certain to shear off the electoral votes of the Deep South. The McCarthy-Kennedy wing was enraged over how Mayor Richard Daley’s cops had beaten the protesters. The nation had seen a convention where Democratic delegates cursed one another on the floor as their partisans brawled with police in the streets.

I came back from Chicago and told Nixon that we should side with Daley and the cops. Nixon’s first campaign stop that fall was a motorcade through downtown Chicago, where huge crowds cheered him.

... And the convention was marred by violence outside, as Mayor Richard J. Daley’s forces cracked down violently on protesters. Photo: Bettmann Archive/Getty Images

The Gallup poll in September had Nixon at 43, Humphrey at 28, Wallace at 21. At every campaign stop, Humphrey was shouted down with chants of “Dump the Hump!”, until he came close to breaking down, denouncing his tormentors as “fascists.”

Desperate, Humphrey rolled the dice on Sept. 30 and pledged to halt all U.S. bombing of North Vietnam. The impact was immediate. The heckling and abuse subsided. He began a steady ascent in the polls. His optimism returned, and he staged one of the great comebacks in presidential politics.

Then he caught a break. On Oct. 3, Wallace introduced his running mate, Gen. Curtis LeMay, who had led the firebombing of Tokyo and who told a stunned press that we Americans have “a phobia about nuclear weapons.” To achieve victory in Vietnam, LeMay said, “I would use anything…including nuclear weapons.” Wallace’s voters began to abandon him and move back home to the Democratic Party.

“ The Cold War consensus that had existed from the Berlin blockade of 1948 through the Cuban missile crisis was no more. ”

The election ended in a virtual tie, with both candidates receiving roughly 43% of the popular vote. But Nixon had won in the electoral college and was now president-elect of the United States.

What had 1968 wrought?

The American establishment, “the best and the brightest,” had been broken on the wheel of Vietnam. Liberal elites would move to ally themselves with the antiwar left and to denounce as “Nixon’s war” the cause into which they themselves had led the country.

The Cold War consensus that had existed from the Berlin blockade of 1948 through the Cuban missile crisis was no more. The Democratic candidate in 1972 would run on the slogan “Come home, America!” Foreign policy leadership passed from the party of Truman and Kennedy to the party of Nixon and Reagan. After 1968, the word “victory” was rarely heard. The goal now in Vietnam was “peace with honor” or “an end to the war.”

Massive civil disobedience and violent protests would become the new normal. Failed and frustrated extremists would turn to bombings and terrorism. Nixon and Vice President Spiro Agnew would use the radical left and its media enablers as foils to drive a wedge right through FDR’s Democratic coalition, with Nixon calling out his “Great Silent Majority” and Agnew tabling the issue of press power and media bias.

Nixon would be re-elected in 1972 in a 49-state landslide. In four of the five presidential elections after 1968, Nixon’s new majority would crush the Democratic Party. By 1970, six years after Goldwater’s defeat, twice as many Americans would call themselves conservatives as liberals.

As the political wars of 1968 turned American politics upside down, a cultural war had broken out as well. Moral and social issues—abortion, affirmative action, busing, crime, drugs, feminism, gay rights—would tear apart families, communities and the entire nation. The culture wars had begun.

We are another country now, another people. The unity we knew in the Eisenhower-Kennedy era is gone. 1968 was the great divide. 1968 was the turning point.

Mr. Buchanan, a former presidential candidate, served as an aide to Richard Nixon from January 1966 to August 1974. His books “The Greatest Comeback” and “Nixon’s White House Wars” describe those years.