The war of words between President Trump and the North Korean leader Kim Jong-un over Pyongyang’s nuclear program has rattled nerves around the world. But the trial of two women in Malaysia for using the nerve agent VX to kill Mr. Kim’s half brother is a reminder that North Korea’s lethal arsenal isn’t limited to nuclear weapons. The North’s chemical weapons pose a grave risk to South Korea and to regional stability.

Experts say chemical munitions have long been deployed along the demilitarized zone that separates the North and South. In the event of a military attack against the North, analysts say, the regime sees chemicals as an option for a first response. Seoul and its 10 million inhabitants could be hit immediately.

Estimates of casualties are staggering. Images from Syria of children gassed with sarin in recent years have horrified the world; imagine a death toll in South Korea a thousand times larger.

In a June article in Bulletin of the Atomic Scientists, the military historian Reid Kirby used the term “sea of sarin” to describe such an attack. Accounting for sarin’s toxicity, the types of artillery along the DMZ, and vulnerability of children and the elderly, he estimated that a sarin attack could kill as many as 2.5 million people in Seoul and injure millions more. There are about 24,000 United States military personnel in South Korea, along with their families, and thousands of American expatriates.

No one outside of the North Korean government knows with certainty the composition of the country’s chemical stockpile, but intelligence from defectors and the South Korean government suggests that Pyongyang has 2,500 to 5,000 metric tons of some 20 chemical warfare agents.

Experts have long suspected that the North’s arsenal includes VX, which is far more toxic than sarin. The gruesome murder in February of Mr. Kim’s estranged half brother, Kim Jong-nam — a brazen killing in a Malaysian airport that prosecutors say was carried out by two women, trained by North Korean agents, who rubbed the victim’s face with VX — has dispelled any doubts that the North has the chemical. Unlike sarin, VX is “persistent,” meaning it lingers in the environment rather than disperses. The mustard gas that drenched World War I battlefields is the original persistent chemical.

Kim Jong-nam’s murder also raises the specter of new proliferation of chemical arms. The fact that VX made its way out of North Korea undetected to another country could indicate that Mr. Kim would use the North’s extensive criminal smuggling networks to secretly assist other nations, or nonstate actors, in obtaining or producing it.

The chemical menace from North Korea recalls a time when chemical weapons posed far more of a global threat than they do today. The doctrine of strategic deterrence — maintaining stockpiles of powerful weapons to keep a similarly armed adversary in check — didn’t begin with nuclear weapons; rather, it emerged from gas warfare in World War I.

In the war’s aftermath, the Geneva Protocol of 1925 banned chemical warfare but not chemical weapons. The rationale for that apparent contradiction was that rogue nations might disregard international law and use chemical arms to attack cities and civilian populations in much the way North Korea threatens to do today. The way to deter such attacks, the logic went, was to maintain a stockpile. If signatory nations suffered a chemical attack, they could then retaliate in kind. Continue reading.