For South Korea the danger is more immediate. According to physicist David Albright, the founder and president of the Institute for Science and International Security, the North Koreans have between 13 and 30 nuclear weapons and can build as many as five more every year. If Mr. Kim were to detonate one of these bombs in the atmosphere 40 miles above Seoul, it could inflict catastrophic damage on South Korea’s electric power grid, leading to a prolonged blackout that could have deadly consequences.

The United States has 28,500 soldiers, sailors, airmen and Marines in South Korea stationed below the 38th parallel—and more at sea nearby. An electromagnetic pulse attack on South Korea could play havoc with America’s ability to mount an effective response to North Korean aggression. One hopes the troops manning the two already-deployed batteries of the Thaad ballistic-missile defense system are prepared for such a scenario (in a concession to China, the newly elected South Korean government suspended this week the deployment of four additional launchers).

In 2001 Congress established a commission to study the danger of an electromagnetic pulse generated by the detonation of a high-altitude nuclear weapon. It concluded that while there would be no blast effects on the ground, critical electricity-dependent infrastructure could be rendered inoperable. The commission’s chairman, William R. Graham, has noted that several Russian generals told the commissioners in 2004 that the designs for a “super EMP nuclear weapon” had been transferred to North Korea.