North Korea may be the first nation on Earth to develop Submarine launched nukes as a genuine deterrent .
I say that as, for every other nation, they are primarily a first strike system that can be fired at point blank range. (The second strike role in standard MAD doctrine is somewhat shaky at best IMHO).
For North Korea however, this is negates MAD altogether and actually stands it on its head.
As many never tire of pointing out, if Pyongyang initiated the use of Nuclear Weapons it would be a very sudden and final end of the state.
Well whatever you may think of NK's high stakes brinkmanship, it would never have lasted 70 years if it was genuinely insanely, reckless. So I think it is safe to discount first use from the equation.
Second strike/retaliation is more solid but also unlikely as I doubt anyone would really wish to first strike NK either. I have no doubt that any military intervention and regime collapse would be conventional and the anticipation would be rapid military/political implosion.
I discount a strike agianst advancing invaders from Government held territory as it simply puts the first strike scenario back in the mid game rather than opening.
What I do see with Sub launched missiles is a genuine deterrence that can be saved and used after a successful invasion and occupation. This is no less than NK being able to launch point blank attacks at key regional targets, once all its territory has been occupied and against which no retaliatory strike will be possible.
That to me, would be a real deterrence!
I'm not sure how robust and survivable NK SSBs can be to make this latest development a real game changer.
If anything, concentrating NK's small nuclear arsenal on a few short ranged, slow and noisy subs will almost certainly reduce their survivability compared to being hidden deep in mountain bunkers where even nuclear warheads cannot reliably get at them.
I am also not so sanguine about NK not using nukes first if invaded.
I can easily see chubby Kim ordering a nuclear strike on foreign forces on the open seas or even on NK soil, thereby removing the immediate existential threat while not necessarily triggering the automatic nuclear retaliatory strike as would happen if they nuked another country.
If he does that to demonstrate that he has working nukes and is bonkers enough to actually use them, and threaten to fire off all he has left at the home territory of whoever invaded NK, well, that may just be enough to give people enough pause to no want to risk sharing the walk to hell with Chubbs and call it a draw.
Lets remember that for neighbours like China, SK and even Japan and Russia, turning NK into a glow in the dark glass field is almost certain to result in devastating nuclear fallout making vast parts of their territory uninhabitable. So even if NK did shoot nukes first, retaliating with overwhelming nuclear strikes is going to be one hell of a pyrrhic victory.
For that reason, even if NK did use nukes first, all those neighbouring countries would likely apply all the pressure they can to ensure the US doesn't go crazy with the retaliatory nuclear strikes, and would likely insist they be highly limited tactical strikes carried out only to take out remaining NK nukes.