Kim Jong-un’s self-imposed 18-month moratorium on missile and nuclear tests is about to expire at year’s end. Unless sanctions are removed by then, the North Korean dictator is threatening a “Christmas gift” to accompany his traditional New Year speech.

Will he go nuclear with a bigger, better bomb? Test a ballistic missile that can hit America’s East Coast? Launch a satellite to display tech wizardry? Hit Japan? Do nothing and present it as a “concession” to be reciprocated by America?

Either way, the late December fireworks will likely mark an end to President Trump’s diplomatic opening towards a “very good friend.” After a public bromance complete with “love” letters, summits, and a historic crossing of the 38th parallel, Mr. Trump is back to calling Kim “Rocket Man,” and Pyongyang’s propaganda press is again insulting the “dotard” American president.

To his credit, Mr. Trump’s unconventional, highly personal diplomacy managed to suspend, for a while, the North’s endless string of threats against its neighbors and everyone else. Yet, the tender treatment of the tyrant now seems to have run its course.

Thankfully, allies aren’t panicking. “The North Koreans have done nothing to merit the lifting of sanctions,” said a European diplomat this week, after China and Russia proposed a United Nations Security Council resolution to do just that.

Speaking in Seoul, President Trump’s point man on North Korea, Stephen Biegun, dismissed Mr. Kim’s New Year’s deadline, calling instead for renewed denuclearization talks. “We are here, and you know how to reach us,” he said.

America will likely “calibrate” its response according to Mr. Kim’s new year provocation, one diplomat told me. But a measured reaction won’t change minds in Pyongyang. Moreover, it leaves all the initiative in Mr. Kim’s hands.

There’s a better strategy: Make Mr. Kim worry about his hold on power.

By presiding over the world’s most totalitarian state and shutting off the outside world, Mr. Kim currently feels too secure in his role as dynastic ruler. Anyone openly opposing him disappears or is brutally executed. To maintain his near-god status, though, even Kim Jong-un needs political backing at home.

A senior fellow at the Foundation for the Defense of Democracies, David Maxwell, who specializes in Korean affairs, says America can shake Mr. Kim’s confidence by highlighting his horrific human-rights record. “When we talk about missiles and nuclear bombs, it reinforces him domestically,” he says, while the human-rights issue “weakens him.”

More and more information from the outside world nowadays manages to get to North Koreans, Mr. Maxwell says. Thumb drives, tape cassettes, and other forms of media are smuggled in while ever-stronger radio signals from South Korea beam broadcasts.

Defectors, including some former high-ups, also help. America should encourage them to speak up and amp up efforts to transmit their stories to the North. After living in Seoul or the United States, they know what freedom looks like, and they can describe the experience to their still rights-deprived brethren.

Mr. Trump, evidently, isn’t that eager to talk about human rights. Earlier this month his ambassador at the United Nations, Kelly Craft, blocked — for a second year in a row — a Security Council session on North Korea’s human rights. Instead, the council condemned secret testing of ballistic missile parts and other arms violations.

President Trump’s former national security advisor, John Bolton, publicly rebuked the move. “Maximum pressure against North Korea’s nuclear program requires mobilizing the widest possible support,” he tweeted. “Kim’s repression of his people, terrorist activities and pursuit of WMD’s all warrant the fullest scrutiny. We should take the lead, not obstruct other nations.”

Mr. Bolton has long advocated regime change in North Korea. His detractors counter that too much pressure risks hard pushback: If Mr. Kim fears the loss of his throne, he’d likely go ballistic and try to destroy Seoul, Tokyo, Seattle or all of the above.

Mr. Kim, though, won’t risk total annihilation of his country and his regime. America’s best option, therefore, is to ratchet up the pressure, do all we can to undermine Mr. Kim’s power and then give him the option: Negotiate seriously or lose it all.

If the tyrant becomes so discredited that North Koreans or their Chinese neighbors have no choice but to push him out of power, well, who’d miss him?

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From the New York Post. Image: Detail from Elliott Banfield’s cartoon of Ambassador Bolton at the United Nations; the cartoon was originally published in the Sun in 2006.