They wouldn’t attempt to attack any city, unless it was a *Gotterdammerung *(my auto-correct rolled over and died trying to parse that) situation or it was an accident. If an accident, then who knows where or when? Perhaps a brief initial strike somewhere non-inhabited in South Korea, to prove to the U.S. et al that they were serious (well, seriously starving), and to make with the aid already, or things would get worse?
If a Gotterdammerung, then I fully believe that psychopath would give the orders for as many people as possible to join him in Hell. How many would that be depends on how many missiles he has (I’m surprised he did another test w/o having a non-interdictable deterrent already in place. I’m also surprised we haven’t seen a multi-stage weapon’s test yet.), divided by how many of his soldiers would actually push the button, and divided by how many of the damned things are likely to work.
Say 50 road-mobile storable-liquid-fueled ICBMs. Say 2/3 try to launch, and say 1/2 of those work. That’s 16 warheads (no sign they know how to MIRV yet), each about 20 kT, maybe less, maybe as much as 50kT. South Korea can get thumped by other non-ICBM means, so pick on Japan, the U.S., and hell, that crazy bastard may try China and Russia too. After all, it’s their fault too for not giving him enough aid, right?
Anyway, generate a countervalue (he’s only got a few weapons; he’s not wasting them on US military sites) list for 16 atomic weapons striking at least the first two countries, and then whoever else grabs your fancy. Shrink it by however many you think Aegis Ashore, THAAD, Fort Greeley, etc… can attrit.
Likelihood? What’s the likelihood that Un thinks his society will fall and try to kill him by 2020? There’s always the chance of a “Whoops!” when sabre-rattling though.
I think the chance is small, but I also think then that in order for that to happen, NK will be among the leaders of foreign aid from the U.S. Is it worth what we give Israel and Egypt each year (or Pakistan or whoever else is the lucky winner at the Department of State Fair) to keep NK afloat, Un calm, and the missiles stilled?
Consider further that an atomic weapon detonating in a US city will be the largest catastrophe by at least an order of magnitude in this country’s history. So, the Galveston Storm of 1900? 10x the casualty toll from that. What would the economic costs be? 10x 9/11? 100? Danegeld doesn’t seem so bad, now, does it?