short-range nuclear missiles aimed at South Korea and vowed to improve them by making the warheads “smaller, lighter and tactical.” South Korea’s strategy of deterrence has been based on the belief that the best chance it has against the North without nuclear weapons of its own is to build up a conventional missile defense and deploy ever more powerful “bunker busters” to make Mr. Kim fear for his life.

When North Korea tested its intercontinental ballistic missile in 2017, the United States and South Korea responded by launching their own ballistic missiles to demonstrate their “deep-strike precision” capabilities. In his book “Rage,” the journalist Bob Woodward wrote that the American missile traveled the exact distance between its launching point and the location from which Mr. Kim watched his I.C.B.M. launch.

collapsed, North Korea resumed tests in 2019, rolling out three short-range ballistic missiles that were designed to counter the allies’ antimissile capabilities.

North Korea’s old fleet of Scud and Rodong missiles used liquid fuel and lacked precision. The country’s new generation of missiles uses solid propellants, making them quicker to launch, easier to transport and more difficult to target. They also have greater accuracy and evasive maneuvering power that could confound the South’s missile defense systems.

The new solid-fuel ballistic missile North Korea tested in March likely evaded the allies’ radar during its low-altitude maneuvering, leading the South Korean military to estimate its range at 280 miles, not the 372 miles the North claimed, said Chang Young-keun, a missile expert at Korea Aerospace University. Mr. Chang said the missile could also likely increase range and warhead weight because it was powered by “the largest solid-fuel rocket motor developed and tested in North Korea so far.”

The North’s ICBMs still use liquid fuel, which takes hours to load before launching, making them vulnerable to American pre-emptive strikes. But in his January speech, Mr. Kim vowed to build solid-fuel ICBMs, presenting an even bigger challenge for American missile defenses. Such prospects deepen the fear among some South Koreans that Washington would be less likely to intervene if it, too, faced a possible North Korean nuclear attack.

View Source