North Korea has just tested a nuclear bomb. But it has tested five nuclear bombs before. It has demonstrated that it is an emerging nuclear-weapons power, but then again we already knew that. It has tested America’s largely untested president with the stuff of nightmares, but Donald Trump has already confronted several North Korean ballistic-missile tests—including one over Japanese territory and two involving missiles thought capable of reaching the United States. So why is North Korea’s latest nuclear test significant?

A nuclear test isn’t only an advertisement of military advances. It isn’t only a means of intimidating enemies and impressing allies. It is also—and primarily—a test in the scientific sense of the term: You don’t waste limited and valuable fissile material on a mere political provocation.

The North Koreans are “not just testing to say something—there’s a reason for testing,” Joel Wit, an expert on North Korea’s nuclear program, told me. “Any country that builds nuclear weapons proceeds down predictable technological paths.” With every trial of a weapon, “they’re trying to learn something new.”

In this case, North Korea may be experimenting with how to increase “yields,” or the amount of energy released when a nuclear weapon is detonated. Kim Jong Un’s government said Sunday’s test involved a hydrogen bomb, a two-stage device that combines nuclear fission and fusion reactions to generate much higher yields than atomic weapons, which rely only on nuclear fission. (The Kim regime has claimed to possess a hydrogen bomb since late 2015, though these claims are difficult to verify and had previously been met with skepticism. Hydrogen bombs have never been used in warfare.)

Sunday’s nuclear test seems to have been North Korea’s most powerful by far—perhaps several times more powerful than its fifth nuclear test last year, according to South Korean and Japanese officials. A Norwegian earthquake-monitoring agency has estimated the yield at 120 kilotons; the atomic weapons that the United States dropped on Hiroshima and Nagasaki during World War II were 15 kilotons and 20 kilotons, respectively.

As U.S. intelligence officials reportedly suspect, North Korea may also be fine-tuning small, light nuclear warheads that can be placed on missiles capable of hitting not just U.S. allies such as South Korea and Japan, but potentially the U.S. mainland. North Korean state media has stated that it can mount a hydrogen bomb on an intercontinental ballistic missile (ICBM), though it has provided scant evidence for this beyond releasing pictures of Kim Jong Un inspecting an apparent nuclear device.

The test “gives more credence to [North Korea’s] claims that they’re developing a hydrogen bomb,” though it doesn’t confirm that the North now possesses a functional one, Wit told me. “The higher the yield you have, the less need for accuracy. … The ability to create damage even if you’re off course increases a lot.”

Put another way, the “reason for continued [nuclear] testing is to reduce the size and mass of the warhead—in order to be able to mount these on ballistic missiles of increased range,” Siegfried Hecker, a nuclear-security expert who has examined North Korea’s nuclear facilities several times, told me by email. “Testing is also required to reduce the amount of fissile materials needed for the warheads.”

That North Korea has now conducted six increasingly powerful nuclear tests—four since Kim Jong Un came to power in 2012—and a flurry of missile tests in recent years suggests that its stockpile of nuclear weapons and systems to deliver them are expanding.

But, according to Wit, these developments do not necessarily mean that Kim Jong Un is more aggressive than his father or grandfather. Instead, the heightened activity may simply be representative of the North Korean nuclear program’s maturation. North Korea has spaced out its nuclear tests more than nuclear powers like Pakistan and India—so far, it has conducted six tests in 11 years—which may have helped the Kim regime do more in-depth analyses of what went right and wrong with each experiment.


North Korea’s Escalating Nuclear and Missile Tests

Reuters

As the arms-control expert Jeffrey Lewis wrote in The Atlantic after North Korea’s fifth nuclear test last fall, by their fifth nuclear tests, Britain, China, France, Russia, and the United States, were all on their way to building thermonuclear bombs—the same category of weaponry that North Korea claims to have demonstrated this week—and were widely recognized in the world as nuclear powers. Why judge North Korea any differently?

The latest nuclear test also vividly illustrates the limits of China’s influence on North Korea, despite the fact that it is North Korea’s largest trading partner and most important ally. As hostilities have grown between North Korea and the United States, Beijing has warned that tensions on the Korean peninsula are reaching “a tipping point” and, under pressure from the Trump administration, signed on to tough United Nations Security Council sanctions against Pyongyang.

Yet, Kim Jong Un just went ahead with a nuclear test anyway. U.S. officials tend to “grossly underestimate” North Korea’s determination to press ahead with its nuclear-weapons program even when pressured by its only (semi-)friend in the world. What’s more, nuclear-weapons program typically “move forward quicker than the effect of sanctions,” he added.

North Korea’s most recent demonstration of its nuclear prowess shows how such weapons programs take on a certain technological and political momentum. “[E]very time … they start testing an ICBM or they conduct this nuclear test, everyone acts shocked and dismayed. It’s about time we stopped acting shocked and dismayed and started thinking about what to really do about this besides condemning North Korea.” What to do, he explained, could include initiating a diplomatic dialogue with North Korea without preconditions. It could also include mobilizing more U.S. military assets in the region, including possibly tactical nuclear weapons, to better deter North Korean aggression.

On Sunday, Donald Trump ominously tweeted that “appeasement” won’t work with North Korea and that the North Koreans “only understand one thing,” but Wit argued that “U.S. military threats ring increasingly hollow to the North Koreans” because they know U.S. officials are aware of the catastrophic human toll that conflict on the Korean peninsula could take. “Tweets don’t do it. … The initial uncertainty the North Koreans may have had about President Trump and whether he was really a serious tough guy or not—I think that uncertainty has probably worn away now.”

North Korea’s ultimate goal, Wit said, is to develop a “survivable nuclear deterrent”—a means of securing the Kim regime from external threats posed by the United States and its allies. And a survivable nuclear deterrent “is a flexible thing. It’s not as if there’s some handbook you go to. … Every country that builds nuclear weapons addresses an issue, which is: How much is enough?” If U.S. leaders want to limit the threat of a nuclear-armed North Korea, they must “influence the North Korean calculation about how much is enough.”

“What the Trump administration should do now is to focus on convincing North Korea not to use a nuclear weapon on the Korean peninsula—that is the nuclear crisis today,” Hecker said. It must be made clear that North Korea’s use of a nuke would “cause unacceptable damage from a U.S. perspective and annihilation of the Kim regime from a North Korean perspective.”

Pressuring the North Koreans to give up their nuclear weapons altogether, Hecker added, “is a much longer-term proposition.”