Technically, North Korea doesn't have nuclear missiles; they have have a nuclear bomb design that they've tested, (albeit a speculated "fizzle," as the test yields were far below that of other country's early nuke tests) and they also have some ballistic missile technology.
However, building a nuke is one thing; making one small and durable enough to fit in the tip of a missile and survive the searing heat of a >10,000MPH atmospheric re-entry is another. There's also a question of range; N.K. has no ICBMs; their proposed, bulky Taepodong-2 missile has consistently had a spectacular failure on each of the tests they've made, and even if it worked, it might only BARELY classify as "intercontinental;" it could potentially reach the Aleutian Islands and some parts of Alaska, but be far, far from being able to reach the rest of the USA.
Still, a strong, functional anti-missile defense is important, as if the USA manages to build a combined system that can render nuclear missiles useless, then no countries will bother trying to make them anymore; America will have succeeded in making nukes obsolete, hence eliminating the shadow of potential nuclear war...
So really, is perhaps a try for world peace (or at least, a world free of nuclear weapons) not as important as hare-brained schemes to produce "green" energy that actually wind up yielding more pollution than more conventional means?