TOKYO • If the US were to strike North Korea, Mr Kim Jong Un's regime would retaliate by unleashing its conventional weaponry lined up on the demilitarised zone (DMZ) that has separated the two Koreas for about seven decades.
And that conventional weaponry is reliable, unlike North Korea's missiles, and could cause major devastation in South Korea, which is a staunch ally of the United States.
"This becomes a very limiting factor for the US," said retired air force officer Carl Baker, who has extensive experience in South Korea.
As tensions between Pyongyang and the outside world have risen over the past month, there has been more talk about Washington using military force either to pre-empt a provocation or to respond to one.
Although most of the recent focus has been on North Korea's ambition to be able to strike mainland US with a missile, the South Koreans have been living under the constant threat of a conventional attack from the North for decades.
North Korea has "a tremendous amount of artillery" right opposite Seoul, saidMr Joseph Bermudez Jr, a senior imagery analyst at 38 North, a website focused on North Korea.
The Second Corps of the Korean People's Army based at Kaesong on the northern side of the DMZ has about 500 artillery pieces, Mr Bermudez said. And this is just one army corps; similar corps are on either side of it.
All the artillery pieces in the Second Corps can reach the northern outskirts of the South Korean capital, just 50km from the DMZ, but the largest projectiles could fly to the south of Seoul. About 25 million people - or half of the South Korean population - live in the greater Seoul metropolitan area.
About half of North Korea's artillery pieces are multiple rocket launchers, including 18 to 36 of the huge 300mm launchers that Pyongyang has bragged about. State media last year published photos of the system during a test firing that Mr Kim attended.
The 300mm guns could probably fire eight rounds every 15 minutes, Mr Bermudez said, and have a range of about 70km.
"This could do a lot of damage," he said. "If they hit a high-rise building with a couple of rounds of artillery, people get into their cars, causing huge traffic jams, so North Korea could target highways and bridges in cascades."
If North Korea were to start unleashing its artillery on the South, it would be able to fire about 4,000 rounds an hour, the Nautilus Institute's Mr Roger Cavazos said in a 2012 study. There would be 2,811 fatalities in the initial volley and 64,000 people could be killed that first day, most of them in the first three hours, he wrote.
Some of the victims would be American because the US military has about 28,000 troops in South Korea. This prospect of massive damage and casualties has restrained successive US administrations, however provocative North Korea has been.
"Every US administration, as they have looked at this problem, has said that all options are available. But that's not really true," said Mr Baker, who is now at the Pacific Forum of the Centre for Strategic and International Studies. "We really don't have a military option."