Like Israel actually, North Korea runs one of the most competent smuggling groups in the world. They smuggle drugs into Australia and Japan regularly, and possess all the resources needed to infiltrate borders of most of their neighbours with ease. In spite of what we think about what a 'traditional nuclear strike' would/should look like, the fact is that a two-man team carrying a portable nuclear device is quite a valid threat to a lot of modern western democracies, and this is quite well understood by most governments. (In fact, its the basis of Israels' own investment in nuclear armament: the David option ..)

So, don't just consider the ideal that nuclear weapons have to be delivered by missile. They can also be delivered by donkey.