Load it in NK, smuggle it into China via bribes/mini sub chicanery/other NK special forces voodoo. Thence to an international air freight terminal. Load onto 747 freighter via more of the same, fly to US, airburst over desired target. Use some of the same illicit pathways that NK allegedly uses to ship out their methamphetamine throughout East Asia. Not at all easy or reliable, and Holy Shit if their shipment got intercepted in the middle of all of that, but the Kims had means before their (and Yuzhmash’s) rocketry achievements of potentially getting a warhead to the States.
Easier to just give the order to road-mobile ICBM crews to fuel, elevate and launch their birds—they can’t move the missiles around while they’re fueled, right?
Anyway, none of this is going to happen unless Un, or his OGD backers that actually have positive communication and control over the weapons, think it’s a Götterdämmerung situation for them, they’ve no way out, they and their families will die anyway, horribly, and they’re psycho enough to take a few million innocents with them. Yet that’s still enough of a Very Bad Day for the US if they did, that a lot of otherwise unthinkable actions become thinkable.
Aside, on the NKs potentially selling what’s left of ISIS a device, if they did—they wouldn’t—and if it went off, how could we know for certain that the explosion was caused by a NK device, with NK nuclear material? As opposed to some other design with NK material, or a NK device with ex-Soviet material, and so on? I mean, I doubt the West has material coupons from any NK reactor and reprocessing runs, and I doubt the West has device design documents in sufficient detail to model for certain what reaction products would result from such a device. Though samples have been obtained from previous NK nuclear tests, and that’d help considerably. Provided the sold device was made the same way, from the same material, as the device that was tested.
The point is, if Seattle or Seoul or Osaka or Riyadh or Beijing disappears in a flash of light, with no rocket trail helpfully leading back to North Korea, how would people know who to blame?