The documents reveal a long-secret set of preparations by the commander, Gen. William C. Westmoreland, to have nuclear weapons at hand should American forces find themselves on the brink of defeat at Khe Sanh, one of the fiercest battles of the war.

With the approval of the American commander in the Pacific, General Westmoreland had put together a secret operation, code-named Fracture Jaw, that included moving nuclear weapons into South Vietnam so that they could be used on short notice against North Vietnamese troops.

Johnson’s national security adviser, Walt W. Rostow, alerted the president in a memorandum on White House stationery.

The president rejected the plan, and ordered a turnaround, according to Tom Johnson, then a young special assistant to the president and note-taker at the meetings on the issue, which were held in the family dining room on the second floor of the White House.

“When he learned that the planning had been set in motion, he was extraordinarily upset and forcefully sent word through Rostow, and I think directly to Westmoreland, to shut it down,” Mr. Johnson said in an interview.

He said the president’s fear was “a wider war” in which the Chinese would enter the fray, as they had in Korea in 1950.

“Johnson never fully trusted his generals,” said Mr. Johnson, who is of no relation to the president. “He had great admiration for General Westmoreland, but he didn’t want his generals to run the war.”

Had the weapons been used, it would have added to the horrors of one of the most tumultuous and violent years in modern American history. Johnson announced weeks later that he would not run for re-election. The Rev. Dr. Martin Luther King Jr. and Robert F. Kennedy were assassinated shortly thereafter.

The story of how close the United States came to reaching for nuclear weapons in Vietnam, 23 years after the atomic bombings of Hiroshima and Nagasaki forced Japan to surrender, is contained in “Presidents of War,” a coming book by Michael Beschloss, the presidential historian.

“Johnson certainly made serious mistakes in waging the Vietnam War,” said Mr. Beschloss, who found the documents during his research for the book. “But we have to thank him for making sure that there was no chance in early 1968 of that tragic conflict going nuclear.”

The new documents — some of which were quietly declassified two years ago — suggest it was moving in that direction.

With the Khe Sanh battle on the horizon, Johnson pressed his commanders to make sure the United States did not suffer an embarrassing defeat — one that would have proved to be a political disaster and a personal humiliation.

The North Vietnamese forces were using everything they had against two regiments of United States Marines and a comparatively small number of South Vietnamese troops.

While publicly expressing confidence in the outcome of the battle at Khe Sanh, General Westmoreland was also privately organizing a group to meet in Okinawa to plan how to move nuclear weapons into the South — and how they might be used against the North Vietnamese forces. Editors’ Picks The Towers Came Down, and With Them the Promise of Public Housing What Makes ‘The Good Place’ So Good? Everything You Need to Know for the Midterm Elections

“Oplan Fracture Jaw has been approved by me,” General Westmoreland wrote to Adm. Ulysses S. Grant Sharp Jr., the American commander in the Pacific, on Feb. 10, 1968. (The admiral was named for the Civil War general and president, who was married to an ancestor.)

The plan did not last long.

That day, Mr. Rostow sent an “eyes only” memorandum to the president, his second in a week warning of the impending plan.

Two days later, Admiral Sharp sent an order to “discontinue all planning for Fracture Jaw” and to place all the planning material, “including messages and correspondence relating thereto, under positive security.”

The incident has echoes for modern times. It was only 14 months ago that President Trump was threatening the use of nuclear weapons against North Korea — which, unlike North Vietnam at the time, possesses its own small nuclear arsenal.

There have been other moments when presidents had to consider, or bluff about, using atomic weapons. The most famous was the Cuban missile crisis in 1962, the closest that the United States and the Soviet Union came to nuclear conflict.

And before he was dismissed in 1951 by President Harry S. Truman, Gen. Douglas MacArthur explored with his superiors the use of nuclear weapons in the Korean War. Truman had feared that MacArthur’s aggressive strategy would set off a larger war with China, but at one point did move atomic warheads to bases in the Pacific, though not to Korea itself.

But the case of Khe Sanh was different, the documents show.

“In Korea, MacArthur did not make a direct appeal to move nuclear weapons into the theater almost immediately,” when it appeared that South Korea might fall to the North’s invasion in 1950, Mr. Beschloss said. “But in Vietnam, Westmoreland was pressuring the president to do exactly that.”

The seriousness of that discussion was revealed in a lengthy cable about the Khe Sanh battle that General Westmoreland sent to the chairman of the Joint Chiefs of Staff, Gen. Earle Wheeler, on Feb. 3, 1968.

“Should the situation in the DMZ area change dramatically, we should be prepared to introduce weapons of greater effectiveness against massed forces,” General Westmoreland wrote in a cable that was declassified in 2014 but did not come to light until Mr. Beschloss cited it in his forthcoming book.

“Under such circumstances, I visualize that either tactical nuclear weapons or chemical agents would be active candidates for employment.”

Within four days, Admiral Sharp, the Pacific commander, wrote that he had “been briefed on the contingency plan for the employement of tactical nuclear weapons in the Khe Sanh/DMZ area which was drafted by members of our respective staffs last week in Okinawa.’’

He declared it “conceptually sound” with some minor alterations, and asked for a full plan to be forwarded to him “on an expedited basis so that the necessary supporting plans can be drawn up.”

Three days later, General Westmoreland wrote back that he had approved the plan. At the White House, Mr. Rostow noted to the president: “There are no nuclear weapons in South Vietnam. Presidential authority would be required to put them there.”

That notification led to the president’s angry eruption, and within days Admiral Sharp, once so eager to develop the plans, ordered a shutdown.

“Discontinue all planning for Fracture Jaw,” he commanded in a Feb. 12, 1968, cable to General Westmoreland, with copies to the Joint Chiefs. “Debrief all personnel with access to this planning project that there can be no disclosure of the content of the plan or knowledge that such planning was either underway or suspended.”

None of this was known to the American Marines and other soldiers who were being shelled at Khe Sanh.

“I don’t remember any discussion of atomic weapons on the ground at Khe Sanh,” Lewis M. Simons, then an Associated Press reporter on the ground with the troops, and later a Pulitzer Prize-winning reporter who worked at The Washington Post and Knight Ridder newspapers.

Mr. Beschloss’s book, which will be published on Tuesday by Crown, examines challenges facing presidents from Thomas Jefferson to George W. Bush. It also reveals that at the same time the nuclear debate was underway, senators were outraged to discover that the president and his aides had misled them about progress in the Vietnam War.

The chairman of the Senate Foreign Relations Committee, J. William Fulbright, Democrat of Arkansas, told his fellow senators that “we were just plain lied to,” and that the lying meant that the United States had lost “a form of democracy,” according to transcripts obtained by Mr. Beschloss, who is a frequent contributor to The New York Times.

There was even discussion of the possibility of impeaching the president for those lies. That discussion was terminated by Johnson’s decision, announced later that spring, not to seek re-election.

By David E. Sanger - The New York Times - October 6, 2018