Many North Korean soldiers have fake wooden guns. North Korea wouldn't give America any real trouble in a war (any more than Afghanistan or Iraq at least) - it'd be the massive influx of refugees to surrounding countries which would cause massive trouble (and be massively destabilising). That's not in America's interest, and it certainly isn't in China's, South Korea's, Russia's, or Japan's.
I suppose there's the risk that North Korea could get off a single nuke, but from what we've seen so far that risk is low, and they'd probably manage to miss any densely populated areas.
North Korea's relative strenght lies in their missile capabilities and in their intelligence services. Besides, modern conflicts are only rarely decided through conventional warfare.
The only real threat from NK lies not in their capabilities but in their allies, namely China. A war would go exactly like the last one, we would push them all the way to Chinese territory and then get hit by a human tidal wave.