Here is what most don't understand. Seoul is within range of 15000 fixed rocket and artillery pieces. They are embedded in bunkers with blast doors that open for firing and close for reloading. Seoul has 25 million living in high density targets. Top that off with North Korea has the largest stock pile of chemical and biological weapons on the planet that can be fired from that artillery. While these are relatively low tech except for their 300 mm rockets with a range of 100 miles, one volley can put 350 metric tons of explosives (plus bio and chemical) into that city. They don't need to be accurate because the targets are so dense and plentiful. We are talking hundreds of thousands in casualties in a short time frame. It would take weeks to destroy their capabilities, Seoul would be decimated. Plus THAAD is useless vs artillery, if we do strike we doom Seoul to destruction.