Why North Korea’s Latest Nuclear Claims Are Raising Alarms

For decades, Washington, Seoul and their allies have tried both negotiations and sanctions to roll back North Korea’s nuclear ambitions and prevent technological advances like these.

Those efforts failed. The world is now seeing what comes next: a bigger, more dangerous nuclear arsenal that poses a greater threat to the United States and its allies in Northeast Asia.

It’s unclear whether or not the warheads in the photo were real. North Korea has a track record of showing mockups of weapons still under development for the sake of propaganda. Still, Mr. Kim has set bold goals for an arms buildup, pouring resources into weapons development.

There is no doubt that the North has achieved major upgrades to its arsenal, including solid-fuel ballistic missiles that can also make mid-air maneuvers. It is also developing cruise missiles, submarine-launched missiles and underwater drones that it says could be nuclear-armed. All these efforts are aimed at making its nuclear attacks more effective.

A failed military spy satellite launch on Wednesday is the latest demonstration of Mr. Kim’s dogged determination to expand the North’s military capabilities.

Smaller warheads that target regional adversaries

Since his failed diplomacy with President Donald J. Trump, Mr. Kim has urged his country to develop “nuclear weapons smaller, lighter and tactical” to target South Korea, Japan and U.S. military bases in the region. These short-range weapons do not threaten the continental United States, but they could boost Mr. Kim’s leverage against Washington by placing American allies under nuclear threat.

Although North Korea has conducted six underground nuclear tests, it has never been clear whether it has successfully developed smaller warheads. The photo released in March was the first concrete indication that it may have succeeded.

So far, North Korea has released photos of three nuclear devices, and the latest one — dubbed Hwasan-31, or “Volcano-31” — is by far the smallest.

Although these are propaganda photos, South Korea in February acknowledged, in a defense white paper, that the North had achieved “a significant level of miniaturization” in its nuclear weapons.

The dramatic shift took only a few years. Mr. Kim had suspended testing missiles and nuclear devices before his 2018 summit with Mr. Trump. He resumed missile tests in 2019 as the talks failed to bring about benefits he had sought, especially the easing of sanctions. Since then, North Korea has been testing short-range ballistic missiles that could serve as delivery systems for these small nuclear warheads at a fast clip.

The resumption of missile tests featured three new short-range ballistic missiles, known to the outside world as KN-23, KN-24 and KN-25. They can be mounted with nuclear warheads and reach South Korea, Japan and U.S. military bases in the region in a matter of minutes. All these characteristics make them more dangerous.

Solid-fuel missiles that are harder to intercept

North Korea’s new series of short-range ballistic missiles are all powered by solid fuel. This makes them easier to hide and transport and faster to launch than older missiles that rely on liquid fuel, which is highly corrosive and unstable.

Compared to liquid-fuel missiles, which could take hours to load prior to launch, solid-fuel missiles can be quickly rolled out of mountain tunnels ready for launch in a short time. This and their shorter burn time make it harder to detect where the missiles are and when they would be launched, thus leaving less time for missile defense systems to react. Their mid-air maneuverability also makes it harder to shoot them down from the sky. It’s forcing the United States and its allies to spend billions of dollars to upgrade their defenses.

“Solid-propelled missiles have shorter burn time than liquid ones. This makes the window of detection, where infrared sensors could see the hot rocket motor working, smaller,” Dr. Markus Schiller, a rocket analyst from the German space technology consulting firm ST Analytics, said.

North Korea plans to retire its old Scud and Rodong liquid-fuel short- and medium-range ballistic missiles, replacing them with a new fleet of solid-fuel missiles, according to the South Korean Defense Ministry.

In December, the North tested a new high-thrust solid-fuel rocket engine, indicating that it is switching its longer-range missiles to solid fuel as well. On April 13, it tested its first solid-fuel ICBM, Hwasong-18, for the first time. Analysts warned that there would be more Hwasong-18 tests as the North tries to perfect the system.

New ways to launch nuclear attacks

Under Mr. Kim, North Korea has also tried to strengthen its military capabilities by developing different methods of launching nuclear attacks.

North Korea has also tested cruise missiles that it said were capable of delivering nuclear warheads. Cruise missiles are slower than ballistic missiles but they can fly at extremely low altitudes and can maneuver between mountains, making them harder to detect by radar.

In early April, the country claimed to have tested “underwater nuclear attack drones” capable of creating “super-scale radioactive tsunamis” to wipe out its enemies’ naval fleets and seaports.

Since 2021, North Korea has tested missiles whose “hypersonic” gliding warheads it says can make mid-air jumps and corkscrew maneuvers.

By diversifying its methods of launching attacks, North Korea is making its nuclear arsenal a greater menace to the United States and its allies.

The remaining technical hurdles

Many of North Korea’s claims about its weapons capabilities, such as those concerning nuclear underwater drones and hypersonic missiles, are exaggerated, according to the South Korean military. Some of its new weapons, such as the Hwasong-18 ICBM, are still in developmental stages.

North Korea has so far launched all its ICBMs at deliberately steep angles, so that they flew extremely high into space but fell into waters between the North and Japan. It has yet to prove that its ICBMs can actually cover an intercontinental range. Another technological hurdle is the so-called re-entry technology. After soaring into space, an ICBM warhead must endure intense heat and friction as it crashes back into the earth’s atmosphere toward its target.

In recent months, North Korea has issued a veiled threat to launch missiles over the Pacific — making the ocean its “shooting range” — to demonstrate that it has mastered re-entry technology and can fire an ICBM on a normal trajectory.

Satellites can help North Korea collect data from such long-range missile tests to improve its ICBM technology. Mr. Kim sees placing satellites into orbit as one of his top priorities, military experts say. The North launched a space vehicle carrying its first military reconnaissance satellite on Wednesday, but it was a failure. The country said it would attempt another launch in the near future after identifying and fixing problems with the rocket and its fuel.

“Although there is a gap between the North’s claims and our assessment, we cannot dismiss them,” Defense Minister Lee Jong-sup of South Korea said in April, referring to the North’s claim to have standardized a small and light nuclear warhead for its various missiles. If North Korea conducts another nuclear test, its seventh, it may be to show that its new and smaller Hwasan-31 nuclear warhead works.

“It will not be easy to say that the North has developed a small and light nuclear warhead until it successfully tests it,” Mr. Lee told South Korea’s legislature.