SEOUL — Four years ago, Kim Jong Un and then-President Donald Trump had an extraordinary summit, the first ever between U.S. and North Korean leaders. They signed an agreement on new peaceful relations and called for a "firm and unwavering commitment to complete denuclearization of the Korean peninsula." Trump declared that he trusted Kim, saying, "I think he wants to get it done. I really feel that very strongly."
But nuclear negotiations between the U.S. and North Korea have stalled since 2019. Pyongyang has rejected Washington's offers of talks without preconditions, and seems intent on sticking to its plan to beef up its arsenal.
When President Biden arrives in South Korea Friday for a two-day summit with his counterpart Yoon Suk Yeol, who was elected in March, the issue of North Korea's nuclear weapons will be high on the agenda. The leaders are facing a North Korea with a more formidable arsenal and more political leverage than before.
Pyongyang has already conducted 16 missile test launches this year. Seoul and Washington are on alert for a possible test launch of additional missiles timed to coincide with Biden's visit.
They are also monitoring for a possible underground nuclear test, which would be the first by North Korea since 2017.
But Pyongyang's advances are not just in developing more lethal weapons — such as new and bigger intercontinental ballistic missiles and hypersonic missiles — but also a more convincing strategy to use them.
"The credibility of the threat due to their capabilities has increased in tandem with the credibility of their intentions," says Jeon Kyung-joo, an expert on the North Korean military at the Korea Institute for Defense Analyses, a government-funded think tank in Seoul.
North Korea's leader Kim Jong Un has repeatedly stated that North Korea would not use its nuclear weapons unless it was attacked. But last month, there was a shift.
"The basic mission of our nuclear forces is to deter a war, but our nuclear weapons can never be limited to the sole mission of deterring war," he said at a military parade. "If any forces try to violate the fundamental interests of our state, our nuclear forces will have to decisively accomplish their unexpected second mission."
What constitutes "fundamental interests" is up to Pyongyang to decide.
Based on previous statements, "the vital national interest for North Korea includes raising questions about North Korea's human rights violations, and even sanctions on North Korea," says Park Won-gon, a North Korea expert at Ewha Woman's University in Seoul.
Washington does not rule out the possibility that North Korea might use nuclear weapons first. Pyongyang is building both short-range nuclear weapons to target U.S. and South Korean forces in Asia, and long-range ones to threaten the U.S. mainland.
Kim's sister, Kim Yo Jong, a powerful official, stated last month that Pyongyang could use its nuclear weapons not as a last resort but at the beginning of a conflict — to demoralize the enemy or simply to conserve conventional military strength.
Jeon Kyong-joo believes it's unlikely that Kim would use nuclear weapons unless he thinks he faces a conflict with U.S. and South Korean forces, which have an advantage in conventional arms. Pyongyang's ultimate goal remains to unify the peninsula under its own rule, she says.
"It remains a very important goal, and one that must be achieved over the very long term," she says. "But they must think that recognition as a nuclear state is a necessary first step towards that goal."
While South Korea does not have nuclear weapons, it has plenty of missiles, some of which may be aimed right at Kim Jong Un.
"South Korea has been very clear that the intention of this force is to decapitate the North Korean leadership in a crisis, which is incredibly escalatory," says Jeffrey Lewis, an arms control expert at the Middlebury Institute of International Studies in Monterey, California. The rival Koreas "both think that they are going to go first" in launching a preemptive strike, he says. "And one of them is wrong about that. That's very destabilizing."
Pyongyang has already scrapped a 2018 moratorium on test-launching intercontinental ballistic missiles. But there is also a moratorium on testing nuclear weapons, and it has not detonated any atomic bombs since then.
That's why Seoul and Washington have their sights fixed on a valley near a North Korean village called Punggye-ri, about 60 miles from the border with China.
"There is a tunnel entrance, and inside that mountain there is a network of tunnels," says Jeffrey Lewis. North Korea has conducted six nuclear tests there since 2006, most recently in 2017.
North Korea partially dismantled Punggye-ri in 2018, the same year it declared its moratorium on nuclear testing and began a year-long period of summit diplomacy.
In March, Lewis looked at satellite pictures and saw signs of construction, suggesting that North Korea could be planning to resume nuclear testing.
Despite those concerns, "there are several hundred meters of granite above those test tunnels," Lewis says, "which is more than enough to contain even very large nuclear explosions."
Underground tests are generally less controversial than those above ground. A nightmare scenario would be if North Korea were to "mount a nuclear warhead on a ballistic missile, fire it over Japan into the Pacific and detonate it above the surface of the water," says Adam Mount, a senior fellow at the Federation of American Scientists. Most experts believe such a scenario is unlikely.
This year, North Korea has tested many kinds of missiles, including long-range ones that could reach the continental U.S. and tactical ones for use on the battlefield. It has also tested submarine-launched, train-launched, hypersonic and cruise missiles.
"Depending on which systems they want to make nuclear-capable, it may require them to develop a generation of new nuclear warheads for those systems," Mount says. This could require a lot of nuclear tests.
North Korea needs to miniaturize its warheads to fit various missiles, Mount and Lewis say. And to make its nuclear deterrent credible, Mount says, it needs to prove that the weapons really work.
"Until Kim Jong Un is sure that the United States believes that he has a functional nuclear deterrent, he may have an incentive to continue to demonstrate that he does. Kim Jong Un has a very strong incentive to convince the U.S. president personally," Mount says.
Such a test would signal the end of North Korea's four-year nuclear testing moratorium. By scrapping it, Lewis says, Pyongyang would be clearing away a major obstacle to its nuclear ambitions.
"When Kim Jong Un agreed to stop testing long-range missiles, and when he agreed to stop exploding nuclear weapons, that was a real constraint — not on his deterrent that he had at that time, but on his ability to build the deterrent that he is about to embark on," Lewis says.
The more missile and nuclear tests Pyongyang conducts, Lewis argues, the more obvious the value of the moratoriums will become.
NPR's Se Eun Gong contributed to this report in Seoul.
Correction: May 20, 2022 12:00 am — An earlier version of this story misspelled Jeon Kyung-joo's name as Jeon Kyong-joo.