What could North Korea possibly do that would have major impact?
Well, an all out assault across the line would result in hundreds of thousands of South Korean, and, possibly, thousands of US casualties. Of course North Korea would lose in the end, but the price would be high for us and, especially, for South Korea.
And then there is the question of how accurate their medium range missiles are. A tactical nuke at near ground level at Fukushima would do a huge amount of damage to Japan, China, Korea, and, indeed, the whole world. Would Kim do that? Judge for yourself.