Nuclear bomb [threat by North Korea]

I’m pretty much of the belief that within thirty seconds - almost literally - anything out of Korea would be shot down.
If by some miracle they could get a detonation over American soil, there would be no North Korea (and probably parts of China, S. Korea) left after the retaliation strike that would inevitably come.

So if you’re worried, don’t be.

Maybe a better question is whether the North Korean strike threats are a decoy against a more dirty bomb type scenario - though Korea is old school, doesn’t seem like their style.

I agree, except:

Odds are anything from N. Korea would misfire or be dead before it got to target.
Odds are they have not miniturized anything to fit on a missile anyway.
Odds are the leadership knows their limitations - they may have the hardened artillery to attack Seoul, but invasion? The moment any element of NK left cover - troops, artillery, tanks - they are totally unprotected sitting ducks for US/SK air strikes.
Do they have a short-range missile to lob 5 tons a hundred miles to hit Seoul? I haven’t heard any rumblings of that, and any attempt to bring such a missile to the launch pad would ring alarm bells everywhere.

As mentioned, this posturing is mainly for domestic consumption, and likely to create a sense of emergency to discourage notions of coups, etc.

If NK seriously crosses the line, odds are the USA would call Beijing to plan options (With Seoul if it’s still there, and Tokyo on the line).

China does not want a US ally on its border. Howevr, if NK falls, the demands for unification would be too big to ignore. Even if it failed and the USA told China “you deal with him” - once the People’s Army ran North Korea, with even a touch less totalitarianism than now, the demands for unification (think Germany) would be uncontainable. Only a home-grown nutbar could maintain the iron-plated border that exists between north and south.

After the big one … The USA would likely tell China, “you deal with him or we will”. The Chinese would have to decide between taking out *Kim-whatever *and then handing the country over to the South, or letting the USA and SK do the job.

China would likely prefer the NK keep things quietly going the way they are now without making waves.

And their bioweapon arsenal?
NK Military defectors have been shown to have received recent smallpox inoculations. That means they have virus available.

Their only chance of getting to the US mainland would be to load a warhead onto one of their subs and hand deliver it to somewhere along the west coast. No launch capability, just sail in as close as possible and push the button. They are reported to have about 60 vessels that could theoretically make the 6000-ish mile trip.

All are diesel/electric so they couldn’t make the entire run submerged, but by surfacing at night, preferably in a storm, they could conceivably avoid detection.

That seems even more pointless. A “dirty bomb” is less dangerous than a normal bomb.

Why not a shipping container, onto a cargo ship, into (say) New York harbour? Might take a little bomb/container smuggling to get it onto a ship, but I imagine that’s not impossible.
Mind you, I’d give the bomb very little chance of working, but…