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.