That's what is keeping N. Korea safe for now, they know the USA can't really do much against them without getting China involved, and, while the USA would most likely win, the cost would be heavy: on the field, in terms of equipment and men, in the nearby territories, if China uses its own nuclear capabilities (S. Korea, Taiwan, Japan), and last but not least, they'd have to commit the troops stationed in Western Europe, leaving it quite defenseless.

SN: we're no threat, except maybe to Spain and Canada, 'cause we have no real army