1. Trump is not an idiot or a fool, no matter what the liberal media claim. North Korea is at most an annoyance to the US, even if it is a real threat to South Korea and Japan. Killing tens of millions people is not something one does without thinking, especially if there are less messy ways to accomplish the same result.
2. Anything bigger than a tactical nuclear weapon would seriously endanger the South Koreans and the Japanese due to radication. Much of the US arsenal designed for “end of the world” situations is not useable.
3. The South Koreans want to take over North Korea with its population and economy intact. That is the best solution from an American perspective as well.
4. One should remember that unlike Japan and Germany in World War II, whose decision to go to war was made by leaders they elected, North Korea never had a free election. The only crime of the North Korean people is they got conquered by Kim and his family. No one elected him. Hitler, on the other hand, was a democratically chosen leader, as were the Japanese and Italian leaders in World War II.