We shouldn’t pre-emptively nuke North Korea for a few reasons;

The nuclear fallout would devastate the area, meaning China and South Korea.
We don’t know where all of their missile sites are, and at least some are underground. If we don’t get all of their missile sites, which we won’t, they will respond with their nuclear arsenal.
China would likely take the side of North Korea, and may even respond with their own nuclear arsenal.
The second we attack North Korea, they’re going to attack Seoul. We’re talking a massive artillery strike.

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