As evidence piles up about North Korea’s nuclear capabilities, some of President Donald Trump’s supporters and outside advisers are urging him to launch a pre-emptive strike on Kim Jong Un’s weapons facilities or the missiles being prepared for launch.

But there’s at least one significant reason why U.S. military leaders would be reluctant to carry out such a strike: North Korea would surely retaliate, and this retaliation could include use of chemical weapons.

The casualties would be unimaginable. Some 23 million people live in the region of Seoul, with parts of the city sitting a mere 35 miles from the North Korean border. Also at risk would be some 150,000 U.S. citizens who live in South Korea, including 29,000 troops stationed there.

“Nuclear weapons are not the only threat,” said Kelsey Davenport, director of nonproliferation policy for the Arms Control Association. “North Korea could respond to a U.S. attack using chemical weapons. That would be devastating.”

North Korea is known to have compiled large stockpiles of nerve agents such as sarin and VX. It could fire these from hidden artillery and missile sites, targeting U.S. military bases in the region and cities such as Seoul and Tokyo.

North Korea started developing chemical weapons in 1961, when the father of the country, Kim Il Sung, issued his “Declaration of Chemicalization” amid rising tensions at that time. North Korea officially denies that it possesses chemical weapons, but according to the Korea Research Institute of Chemical Technology, the country has four military bases equipped with chemical weapons and 11 facilities where such weapons are produced and stored.

A separate analysis in 2011 concluded that North Korea had 2,500 to 5,000 tons of these weapons. While a surprise U.S. strike might be able to eliminate some of these stockpiles, North Korea’s artillery guns are thought to be preloaded with chemical weapons, allowing them to be deployed instantly. Hundreds of these guns are within range of Seoul, or at least parts of the city, many of them buried in mountainsides.

“Compared to the nuclear threat, which involves a finite number of warheads and delivery systems vulnerable to air defenses and antimissile systems, the chemical threat is not as easily negated,” wrote Reid Kirby, a military analyst, recently in the Bulletin of the Atomic Scientists.

Some analysts say that North Korea has purposely exaggerated its chemical-weapons capability, part of a strategy to deter a foreign attack. Chemical weapons decay over time and Joo Seong Ha, a defector from North Korea and a journalist based in Seoul, said the north does not have an effective system for maintaining and replenishing its supplies of agents such as sarin and VX. Continue reading.