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.

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SEOUL, South Korea — 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.

VX and sarin are both potent nerve agents, which act on the nervous system of an organism, preventing muscles from functioning. Both are banned by the Chemical Weapons Convention, but several countries maintain stockpiles.

Syria has allegedly used sarin in its battle against anti-government rebels, and a terrorist group in Japan used homemade VX in a 1995 Tokyo subway attack that killed 13 people and injured thousands.

Daniel Pinkston, a North Korea military specialist at Troy University in Seoul, notes that North Korea most recently used VX in the February assassination of Kim Jong Nam, the outcast half brother of Kim Jong Un, at Kuala Lumpur’s airport.

For decades, the city of Seoul has maintained a civil-defense plan to prepare residents for an attack from the north. More than 3,300 civil-defense evacuation centers are spread across the city, along with 17,500 protective shelters. Both the United States and South Korea have developed smartphone apps for their citizens to aid in an evacuation.

But bombardment of Seoul with conventional artillery would possibly kill tens of thousands of civilians, with numbers higher if chemical weapons were used. “Civilians would suffer much greater casualties than the military, which have protective gear,” Pinkston said.

Every year, the United States and South Korea hold a joint military exercise to prepare for a possible conflict with North Korea. This exercise, which starts Aug. 21 this year, generally include troops donning protective gear to simulate conditions during a chemical attack.

David Maxwell, a retired U.S. Army special forces colonel, says he has little doubt about North Korea’s willingness to use weapons of mass destruction.

“It would use chemical weapons on the first day,” he said. One likely target would be U.S. and South Korean air bases, to disrupt allied air power. “Korean and U.S. forces train for this,” he said. “They train to decontaminate runways and aircraft, so they can continue to launch aircraft and rearm them.”

For the same reason, North Korea might also use chemical weapons on ports and navy bases in South Korea, to prevent resupply of forces during a conflict.

“The north would want to degrade the logistics chain of delivery in the south,” Pinkston said. “Chemical weapons could be one tool to do that. It would also have some shock value that might prevent other countries from entering the conflict on the south’s behalf.”

Any North Korean use of chemical weapons, of course, would bring international condemnation and likely escalate the U.S. response. Still, if North Korea were attacked first and its nuclear deterrent were compromised, analysts have little doubt Pyongyang might turn to the chemical option.

Said Davenport, “There are legitimate concerns that sustained use of chemical weapons in Syria has lowered the threshold for their use elsewhere.”