For Mr. Trump, that reckoning is coming even as his strategy to halt the North’s program remains incomplete and largely unexplained, and as some experts say the very idea of stopping Pyongyang’s efforts is doomed to failure. Mr. Trump’s budget is expected to include more money for antimissile defenses, and officials say he is continuing a cyber- and electronic-warfare effort to sabotage North Korea’s missile launches.
The president’s insistence that he will solve the North Korea problem makes it hard to imagine a shift toward acceptance of its arsenal. But in private, even some of his closest aides have begun to question whether the goal of “complete, verifiable, irreversible disarmament” — the policy of the Obama and Bush administrations — is feasible anymore.
“We need to change the fundamental objective of our policy, because North Korea will never willingly give up its program,” Michael J. Morell, a former deputy director of the C.I.A., and James A. Winnefeld Jr., a retired admiral and a former vice chairman of the Joint Chiefs of Staff, wrote last week on the website The Cipher Brief.
“Washington’s belief that this was possible was a key mistake in our initial policy thinking,” added the two men, experienced hands at countering the North. The United States and China, they argue, should abandon the idea of denuclearizing the Korean Peninsula and turn to old-fashioned deterrence.
Similarly, Robert Einhorn, a former senior State Department nonproliferation expert, writes in a new report for the Brookings Institution that a “dual-track strategy involving both pressure and negotiations” would be more likely to “bring China on board.” The technique is reminiscent of what was used to push Iran into nuclear negotiations. But Mr. Einhorn cautioned that “while the complete denuclearization of North Korea would be the ultimate goal of negotiations, there is virtually no prospect that it could be achieved in the near term.”
The Chinese appear unlikely to make more than token efforts to squeeze North Korea, fearing the repercussions if the regime were to collapse, and Mr. Kim has made it clear that he is not about to negotiate away what he sees as his main protection against being overthrown by the United States and its allies.
“China will either decide to help us with North Korea, or they won’t,” Mr. Trump said in the Financial Times interview. If the Chinese fail to act, he added, “It won’t be good for anyone.”
It is unclear how close North Korea is to constructing a hydrogen bomb. But Siegfried S. Hecker, a Stanford University professor who once directed the Los Alamos weapons laboratory in New Mexico, and has visited the North’s main nuclear complex, said the ad for lithium 6, while surprising, was a reminder that North Korea, though a backward country, was still capable of major technical advances.
“I can’t imagine they’re not working on true thermonuclear weapons,” Dr. Hecker said in an interview.
As Mr. Trump and Mr. Xi meet on Thursday and Friday, Mr. Kim, on the other side of the world, may have a plan of his own for the summit meeting: Satellite photographs suggest he is preparing for a sixth nuclear test. Workers have dug a deep tunnel, which can block radioactive leaks if carefully sealed, leaving intelligence experts struggling to estimate the North’s progress.
American intelligence officials, and their South Korean and Japanese counterparts, are debating whether the next blasts will mark major steps down the road to a true thermonuclear weapon.
The lithium 6 ad is evidence that Mr. Kim is following a road map that the United States drew up back in 1954. That is when it tested its first thermonuclear weapon fueled by the isotope. The blast, code-named Bravo, was the most powerful the United States ever detonated. In minutes, its mushroom cloud rose to a height of 25 miles.
Though difficult to make, hydrogen bombs became the symbol of Cold War power — they are awesomely destructive and relatively cheap. The weapon relies on a small atom bomb, inside a thick metal casing, that works like a match to ignite the hydrogen fuel. For decades, bomb makers have used lithium 6 as a standard way of making hydrogen fuel for nuclear arms.
Last month, two Los Alamos scientists argued that the rocky North Korean test site the United States monitors could confine explosions of up to 282 kilotons — roughly 20 times as strong as the Hiroshima blast. Although a hydrogen bomb can be that powerful, so can large atom bombs. Previously, the largest blasts at the site were in the Hiroshima range.
When Mr. Kim declared last year that the North had set off a hydrogen bomb, there was no evidence to back up the claim, such as enormous shock waves felt around the globe. More likely, experts said, Mr. Kim’s scientists had created a “boosted” atomic bomb in which a tiny bit of thermonuclear fuel resulted in a slightly higher explosive yield but fell well short of a true hydrogen bomb.
“It’s possible that North Korea has already boosted,” said Gregory S. Jones, a scientist at the RAND Corporation who analyzes nuclear issues. Like other experts, he pointed to the nation’s two nuclear blasts last year as possible tests of small boosted arms.
A next logical step would be for the North to turn the material it was advertising online, lithium 6, into a more complex kind of thermonuclear fuel arrangement for a much more powerful bomb. The first Soviet thermonuclear test, in 1953, used that method. It was more than 25 times as strong as the Hiroshima bomb.
“It’s a big step,” Dr. Hecker, the Stanford professor, said of a true hydrogen bomb, adding that it was perhaps beyond the North’s skill. But over all, he said, the North had shown technical savvy in carefully pacing its nuclear tests, suggesting that it would eventually learn the main secrets of nuclear arms.
“They’ve done five tests in 10 years,” he said. “You can learn a lot in that time.”
As for the excess lithium 6, any interested buyers may have a hard time answering the ad.
The street address given in the advertisement does not exist. The phone has been disconnected or no one answers. But if the operation really is being run out of the North Korean Embassy in Beijing, it should not be hard for Mr. Xi to find out: It is about two and a half miles down the road from the compound where he lives.