SEOUL—There is no way North Korea is going to give up its nukes, says Pyongyang’s most prominent defector. And if President Donald Trump doesn’t watch out, Thae Yong Ho predicts, he will slip into the quicksand of a deal that will lead “inevitably” to withdrawal of America’s 28,500 troops from South Korea.

In a meeting with foreign correspondents here, Thae outlined the dangers facing U.S. policy with the emergence of what he called “the Trump doctrine.” It bears a distinct resemblance, he said, to the “Nixon doctrine” that led the U.S. to pull its troops from South Vietnam and finally opened the way for North Vietnamese forces to overrun Saigon nearly half a century ago.

The fact that Trump will be sitting down with Kim Jong Un next week in Hanoi, capital of the Vietnamese regime that destroyed South Vietnamese forces in April 1975, deepened the scary image of looming disaster.

Thae is not a random refugee from the north. He was a career North Korean diplomat who was the second-ranking person in Pyongyang’s London embassy until South Korea’s National Intelligence Service helped him, his wife, and two sons defect in August 2016.

The Vietnam analogy is not random, either. Thae is well versed in Asian history.

The “doctrine” proclaimed by President Richard Nixon in July 1969 laid the groundwork for “Vietnamization” under which the U.S. counted on South Vietnamese forces to fight and win on their own. Four years later, American troops were out of Vietnam. Two years after that, Saigon fell.

What’s different, Thae made clear in a wide-ranging session at the Foreign Correspondents' Club here, is that North Korea now has the threat of nuclear warheads. With the North’s nuclear capabilities at the center of all discussions, Thai doubts if the danger was ever so high in reality as in the propaganda that now gives Kim a decided diplomatic advantage.

“North Korean policy was to escalate the crisis of war to justify nuclear weapons,” said Thae. “Kim Jong Un was able to convince the international community that nuclear war with the U.S. was a highly likely scenario.”

“Unfortunately,” he said, “President Trump fell into this trap,” as seen in his rhetoric at the United Nations, where he proclaimed “the major possibility of war.” That was “a major strategic mistake,” in Thae’s view, the culmination of “a strategy that led the international community to fear nuclear war exactly as Kim wanted.”

“If the U.S. is trying to achieve denuclearization, it will require North Korea to discard its nuclear weapons,” said Thae. But that’s not going to happen for one basic reason: “If they take out nuclear weapons, what else do they have? Nothing.”

In short, Kim made a chump of Trump from the beginning, and that is not likely to change next week.

American and South Korean military officers are inclined to agree with Thae. Their greatest fear is that Trump will make significant concessions to Kim, going even beyond his surprise decision after the June summit in Singapore to cancel joint military exercises.

At the diplomatic level, Kim has been all take and no give. “In the past the international community has criticized the DPRK [North Korea] for its failure to honor agreements,” said Thae, “but there has been no such request to honor the Singapore agreement,” and that was vague to begin with.

At the end of the day, Thae suggested, North Korea is going to be accepted as “a de facto nuclear state,” and that being the case, the U.S. will have to talk about long term “disarmament”—a distinct climbdown from denuclearization, a very long-term prospect, and one likely to weaken South Korea much more than the North because it puts at risk the protection the U.S. has offered Seoul since the 1950s.

On the basis of his own work in the foreign ministry, and ongoing contacts in North Korea, Thae believes North Korean strategists are supremely confident that they can gradually whittle away the U.S. commitment to South Korea’s defense. An end-of-war declaration, as called for by Kim and endorsed by South Korea’s President Moon Jae-in, would be one step on the way.

That would mean “the United Nations no longer has justification to extend its command”—the structure formed in the early days of the Korean War under which U.S. and South Korean forces, joined by contingents from 16 other countries, have defended the South ever since.

Already, said Thae, North and South Korea have agreed on a no-fly zone that may eventually extend to the major U.S. headquarters base at Pyeongtaek, 40 miles south of the line between the two Koreas. “Expansion of that zone will make it inevitable that U.S. forces will have to withdraw,” he said.

Given all the dents the North Koreans have made in Seoul’s defenses through negotiations, Thae said, they “have strong confidence in their ability to drive U.S. forces from South Korea.”