The power of disparity between the United States and North Korea at the present moment is overwhelming. This is a tyrannosaurus rex versus a very loud and angry mouse. The thing we need to remember is that if it chose to, the United States could crush North Korea using conventional weapons. It doesn't need to go nuclear.
The concern for the United States and other countries is the safety and well-being of all the people in South Korea, who are extremely vulnerable, even to a ground attack. You could imagine a nightmare scenario where, with the willingness to lose 100,000 troops, North Korea could get to the centre of Seoul and announce they had a nuclear warhead with them, and had therefore essentially captured the peninsula.