WASHINGTON (NYTIMES) - When Dwight D. Eisenhower wanted to force North Korea into negotiations to end the Korean War, he sent a secret message to Pyongyang and Beijing threatening a nuclear strike if they did not come to the table for talks with Seoul.

After North Korea in 1968 seized a US Navy intelligence ship off its east coast with 83 crew members aboard, Lyndon B. Johnson called for diplomacy to defuse the crisis, and began 11 months of clandestine talks to seal the deal.

Bill Clinton used bland but strong admonitions - backed up by the clear threat of a missile strike - to get Pyongyang to halt its efforts to process plutonium in 1994 and submit to talks on freezing its nuclear programme.

President Donald Trump's incendiary remarks on Tuesday (Aug 7), warning of "fire and fury" if North Korea continued threatening the United States, were a jarring hint that he might be willing to discard a decades-old principle against the pre-emptive use of nuclear weapons.

It also broke with a long tradition of American presidents using strongly worded warnings, carefully calibrated threats and urgent - sometimes secret - diplomacy to quell brewing crises with North Korea.

"We never threatened - nor would we ever have threatened - to use nuclear weapons," said William J. Perry, who was sworn in as defence secretary months before Clinton's 1994 confrontation with North Korea. "Beyond that, I, and many of my military colleagues, believe that it's a big mistake to make empty threats. It weakens your credibility if you don't follow through, and it may stimulate the regime on the other side, which has to be very nervous at this moment, to take some reckless actions."

Instead, when the North Koreans ejected United Nations inspectors from their nuclear research reactor in Yongbyon and declared their intention to process plutonium there, Perry said he and Clinton decided their strategy would be to make clear that the United States would never allow such an action. They would press for diplomacy but plan for a military strike to destroy the reactor if talks failed.

"We wanted to be clear, but we didn't want to be explicit about the threat, although they knew and we knew what the plan was," said Perry, who conceived the plan for a cruise missile strike if such action became necessary.

The North Koreans were publicly defiant, calling Perry a "war maniac" - "you don't forget something like that," he said in an interview - but they also recognised the threat was real. A strategically timed op-ed in The Washington Post by Brent Scowcroft, a former national security adviser to Gerald Ford and George Bush, that recommended a military strike against North Korea of precisely the sort the administration had privately been considering helped make the case to Pyongyang that the Americans were serious.

"I've always believed that Kim Il Sung actually believed that I had planted that story," Perry said, referring to North Korea's leader at the time. "There's no doubt that our threat, which was incidentally buttressed by this op-ed, was very clear and very plain and very concerning to them. Within a few days, we had negotiations."

Eisenhower, too, used the leverage of a possible military strike - to push the North Koreans and the Chinese toward a peace agreement to end the Korean War. Having campaigned on ending the conflict, he spent three days on the Korean Peninsula before his inauguration, touring the battlefront in a trip that he later said solidified his view that the situation had reached a stalemate and could not be sustained.

"He sent a secret message to the North Koreans and the Chinese that he would use atomic bombs against them if they didn't come back to the peace table, and it frightened them into returning to the negotiations, which resulted in a truce that exists to this day," said presidential historian Robert Dallek. "This kind of bluster and bombast that we hear from Donald Trump is not calculated, and it is not how presidents have generally dealt with these threats."

Johnson also sought to dial back tensions with North Korea when a crisis arose that coincided with the height of the Vietnam War, when he could ill afford to delve into another major military confrontation, Dallek said. In January 1968, North Korean gun boats seized the Navy intelligence ship Pueblo, taking the crew of 83 captive.

Dean Rusk, then the secretary of state, said the episode fell into "the category of actions to be construed as an act of war".

"My strong advice to the North Koreans," Rusk said, "is to cool it". Johnson called the seizure of the Pueblo a "wanton and aggressive act", adding that "clearly, this cannot be accepted". But as he took the matter before the UN Security Council, Johnson threatened no military action. He said the United States would "continue using every means available to find a prompt and peaceful solution to the problem," adding that he had taken "precautionary measures to make sure that our military forces are prepared for any contingency that might arise".

What followed was 11 months of secret negotiations between American and North Korean officials, with the United States insisting that the Pueblo and its crew be released and Pyongyang refusing to do so without an official apology from a high-ranking official for what it characterised as the deliberate intrusion into North Korean territory of American spies.

Wary of spooking or provoking the North Koreans as the talks began, Johnson called the captain of the Enterprise, an American aircraft carrier that had been in Japan when the Pueblo was taken and headed north afterward, to ask him to turn around.

As the talks wore on, Johnson would ultimately launch a buildup of US forces on and near the Korean Peninsula, sending an armada to the Sea of Japan and 100 warplanes to South Korea in an unmistakable signal to the North Koreans that the United States was ready to attack. But behind the scenes, the president was taking a diplomatic route.

"On the one hand, he was rattling the sabre pretty loudly. But on the other hand, he launched these secret negotiations with the North Koreans to try to find a solution," said Jack Cheevers, the author of "Act of War", a book about the Pueblo capture. "Johnson came under a lot of fire during that 11 months for not doing enough to get the crew back, but in fact, the secret talks were going on and he did succeed in getting them back."

In the end, diplomacy worked, but only after United States officials engaged in a bizarre manoeuvre: Having a high-ranking American official, Army Major General Gilbert Woodward, submit an apology for "grave acts of espionage" committed by the Pueblo against North Korea, which he signed only after holding a news conference declaring that the document was full of lies.

"This has been a most frustrating episode," Rusk said upon their release. "But the great majority of our people have kept their heads."