Then it all stopped. North Korea conducted its last nuclear test in September 2017 and its last missile launch a month later. Trump has gone from ridiculing Kim as "little rocket man" to saying he "fell in love" with the young dictator.
"The missiles and rockets are no longer flying in every direction, nuclear testing has stopped," Trump told the United Nations General Assembly in September.
The president has pointed to this as evidence that his strategy to defang Kim's government is working.
However, many experts point out that although the eye-catching weapons displays may have ceased, other more subtle parts of North Korea's weapons program continue apace. Tests are only part of the story.
"Kim has not changed his policy ... but claims that he's now moved from research-and-development and onto mass production," said Cristina Varriale, a research fellow at the Royal United Services Institute, a London-based think tank.
North Korea continues to produce fissile material and develop missile bases around the country, according to experts and analysis of detailed satellite images.
This transition from testing to production should come as no surprise — it's exactly what Kim told the world he would do at the beginning of the year.
In his New Year's Day address, Kim claimed that his scientists and engineers had completed all the trial runs they needed. Now, he said bluntly, "the nuclear weapons research sector and the rocket industry should mass-produce nuclear warheads and ballistic missiles."
At the current rate of production, North Korea could have around 100 warheads by 2020 — almost half the size of the U.K.'s stockpile, according to Robert S. Litwak, senior vice president of the Woodrow Wilson International Center for Scholars, a Washington think tank.
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Aside from the pause in testing, Trump also takes credit for North Korea's claim that it destroyed its nuclear testing site at Punggye-ri.
However, this demolition is unconfirmed, reversible and even irrelevant in terms of producing more warheads, experts warn.
North Korea's continued activity is sometimes portrayed as some sort of deception. But even South Korea points out that Kim is not deceiving anyone because he made no concrete promises in the first place.
While Trump and Kim pledged to "work toward complete denuclearization of the Korean Peninsula" at their historic summit in June, their joint declaration was so vague that it has been widely dismissed as almost meaningless.
Just landed - a long trip, but everybody can now feel much safer than the day I took office. There is no longer a Nuclear Threat from North Korea. Meeting with Kim Jong Un was an interesting and very positive experience. North Korea has great potential for the future!
For one, the U.S. and North Korea disagree about what "denuclearization" actually means. Rather than a one-way street, the North has suggested it should involve the U.S. removing its troops from the region and backing away from the nuclear umbrella Washington offers South Korea and Japan.
This ambiguity has led to stalemate in the months since, with American and North Korean negotiators insisting the other be the first to offer concessions.
The North Koreans have appeared reluctant to deal with Trump's subordinates, such as Secretary of State Mike Pompeo. Some believe they would prefer to exploit the president's perceived impulsiveness, like when he gave an on-the-spot concession to Kim that he would halt large war games with South Korea, appearing to blindside his own military.
In recent weeks, the stalled negotiations have threatened to backslide further.
Earlier in December, the U.S. slapped more sanctions on North Korea, this time on three high-profile officials over alleged human rights abuses. In response, Pyongyang warned of a return to the "exchanges of fire" of last year.
Trump and his team say they want something many experts and even the CIA believe is impossible while Kim is still in power: the complete, verifiable and irreversible dismantlement of North Korea's nuclear arsenal.
This holy grail of denuclearization even has its own acronym — CVID.
Many experts and intelligence officials believe North Korea will never willingly give up its nuclear weapons, especially ones that might be able to strike the U.S., because Kim's regime believes they are its best insurance policy against invasion.
This reality has led many observers to urge the U.S. to pursue more modest short-term goals, with the intention of deterring and containing the world's youngest nuclear power.
"Only with this kind of predictable and agreed-upon working quid pro quo will each side stop assuming the intentions of the other are insincere," according to George Lopez, a former member of the United Nations Panel of Experts on North Korean sanctions.
Moon says he's also committed to denuclearization. But instead of Trump's zero-sum game, he has welcomed intermediate steps along the way, building inter-Korean cooperation on the economy and peace.
China has also played its part. The few concessions that North Korea and the U.S. have given up this year look remarkably similar to a proposal by Beijing that Washington and Pyongyang go "freeze for freeze."
This has led to speculation that Kim did a deal in private with Chinese President Xi Jinping.
"I don't think it is entirely coincidental that Kim met with Xi three times in a very short period of time," says Varriale at RUSI.
Although missile and nuclear tests are perhaps not the game-changer Trump suggests, there are some who give the president some credit for stopping them and bringing North Korea to the negotiating table.
Trump has broken the mold in two ways. He appeared more willing than any of his recent predecessors to countenance what would be a devastating war. And he was the first sitting president to meet face-to-face with a North Korean ruler.
"North Korean leaders did not want their country to be bombed so they stopped testing," says Andrei Lankov, a professor at South Korea's Kookmin University.
The subsequent summit in June "changed the U.S.-North Korea dynamic — at least temporarily — reducing tensions and opening up space for diplomacy with both the U.S. and South Korea," says David Wright a co-director at the Union of Concerned Scientists, in Cambridge, Massachusetts.
Trump has been able to turn down the temperature, so the theory goes, but only because he was willing to take it to a potentially catastrophic boiling point. It's worth noting, Wright says, that any goodwill Trump accrued has been squandered since the summit.
Vipin Narang, a politics professor at the Massachusetts Institute of Technology, has another take.
He believes that Trump knows Kim will never denuclearize — and that he doesn't care. This is because stopping missile tests, rather than actual denuclearization itself, may be enough for Trump to claim success on North Korea and enjoy a political win, Narang said.
"Trump probably rightly calculates that so long as the diplomatic process is ongoing, the testing moratorium will as well," he said. "So he has an incentive to keep the reality show going."
This could suit Kim, too. While gridlocked negotiations and halted tests may enable Trump to claim victory, it also allows North Korea to build up its arsenal.
Alexander Smith is a senior reporter for NBC News Digital based in London.