The standoff over North Korea’s nuclear program has long been shaped by the view that the United States has no viable military option to destroy it. Any attempt to do so, many say, would provoke a brutal counterattack against South Korea too bloody and damaging to risk.

Even the most limited strike risks staggering casualties, because North Korea could retaliate with the thousands of artillery pieces it has positioned along its border with the South. Though the arsenal is of limited range and could be destroyed in days, the United States defense secretary, Jim Mattis, recently warned that if North Korea used it, it “would be probably the worst kind of fighting in most people’s lifetimes.”

Beyond that, there is no historical precedent for a military attack aimed at destroying a country’s nuclear arsenal.

The stakes are even higher now. American officials believe North Korea has built as many as a dozen nuclear bombs — perhaps many more — and can mount them on missiles capable of hitting much of Japan and South Korea.

After all, a pre-emptive American attack would very likely fail to wipe out North Korea’s arsenal, because some of the North’s facilities are deep in mountain caves or underground and many of its missiles are hidden on mobile launchers.

The problem, Mr. Cordesman said, is that there are many ways and reasons for each side to escalate the fighting once it begins.
Stopping it would be much more difficult.

North and South Korea, separated by the world’s most heavily armed border, have had more than half a century to prepare for a resumption of the war that was suspended in 1953. While the North’s weaponry is less advanced, the South suffers a distinct geographical disadvantage: Nearly half its population lives within 50 miles of the Demilitarized Zone, including the 10 million people in Seoul, its capital. “You have this massive agglomeration of everything that is important in South Korea — government, business and the huge population — and all of it is in this gigantic megalopolis that starts 30 miles from the border and ends 70 miles from the border,”

North Korea has positioned as many as 8,000 artillery cannons and rocket launchers on its side of the Demilitarized Zone, analysts say, an arsenal capable of raining up to 300,000 rounds on the South in the first hour of a counterattack. That means it can inflict tremendous damage without resorting to weapons of mass destruction.
The North has deployed at least three systems, though, that can reach the Seoul metropolitan area: Koksan 170-millimeter guns and 240-millimeter multiple-rocket launchers capable of hitting the northern suburbs and parts of the city, and 300-millimeter multiple-rocket launchers, which may be able to hit targets beyond Seoul. There are perhaps 1,000 such weapons near the Demilitarized Zone, many hidden in caves, tunnels and bunkers.

The Nautilus Institute study projects 60,000 fatalities in the first full day of a surprise artillery attack on military targets around Seoul, the majority in the first three hours. Casualty estimates for an attack on the civilian population are much higher, with some studies projecting more than 300,000 dead in the opening days.