NOT FOR the first time, a North Korean leader is attempting to command the attention of an incoming U.S. president. In a New Year’s Day address, Kim Jong Un, who has been racing to develop his regime’s nuclear warheads and missiles, claimed it was close to test-launching an intercontinental ballistic missile (ICBM), thereby demonstrating a capacity to strike the United States. President-elect Donald Trump quickly took the bait, tweeting that “it won’t happen” and then blaming China because it “won’t help with North Korea.”

It remains to be seen if Pyongyang will really manage to launch an ICBM. If it does so successfully, the regime will pose a grave threat to the U.S. homeland. Some experts believe that is improbable in the near future. But Mr. Trump’s response — empty promises of stopping it or rhetorical assaults on China — is unlikely to be helpful.

Mr. Kim, who turns 33 on Sunday, has been on a breakneck mission to establish North Korea as a nuclear power capable of deterring the United States. Last year saw a flurry of tests and demonstrations, including the launch of a three-stage rocket carrying a satellite that could, in theory, be reconfigured as an ICBM. Pyongyang has also paraded prototypes of two other three-stage missiles and carried out ground tests of several of their components. In September, it conducted its fifth test of a nuclear warhead .

The young ruler’s haste has had costs beyond the tightening of U.N. sanctions. Seven of the eight tests of its midrange Musudan missile, which might be able to reach Guam, failed. John Schilling, an aviation expert who writes on North Korea for the website 38 North, concluded last month that the new long-range missiles “will require years of flight testing to reach operational status, with no doubt many more catastrophic failures.” He estimated they were unlikely to be operational before 2020.

The Obama administration may believe the threat is more imminent: President Obama raised North Korea with Mr. Trump during their first transition conversation, after which the president-elect sought a rare intelligence briefing. But it is unlikely Mr. Trump was told anything that would justify his pledge that an ICBM test will not happen. The United States lacks the capacity to shoot down such a missile at launch, and the system deployed in Alaska and California to destroy ICBMs in midcourse is unreliable at best. Attempting to preempt a rocket test would require military action that could trigger a devastating war.

Stopping Mr. Kim consequently may require working with China, which supplies North Korea with most of its energy and food, or negotiating with the regime. Both strategies have been tried unsuccesfully by previous U.S. administrations. Mr. Trump may believe that his aggressive approach to China on a range of issues, including trade and U.S. relations with Taiwan, may ultimately provide leverage on North Korea. But inducing Beijing to bring serious pressure to bear on its neighbor will probably be possible only if the United States makes clear that it is prepared to use force to prevent a North Korean ICBM. Failing that, Mr. Trump may have little choice but to try the tactic he hinted at during the campaign: “Eating a hamburger on a conference table” with Mr. Kim and trying to strike a deal.