For N. Korea to have a "success", they only need to get a bomb to the mainland of the USA. I seriously doubt that their rockets have enough accuracy to hit pinpoint targets. Its more likely, therefore, that they would try to attack very large cities.

I agree with the comment above. They can use other means to carry a nuclear weapon. It's conceivable they could put a nuclear bomb in an airliner, a fishing vessel, a cargo boat, or a submarine. This is challenging for our Coast Guard and Navy - they must search everything. But its a much bigger problem for Asia, where N. Korean vessels sail regularly.

A North Korean trawler, with a nuclear bomb hidden aboard, is probably a much bigger threat than an ICBM. Or for that matter, N. Korea could take an old cargo vessel with markings from another country such as China, and use that vessel as a weapon.