You can't really act like NK hasn't posed a threat. I mean, sure, not to the U.S., but NK has had the ability to shell Seoul with conventional weapons with a pretty assured killing of at least a few hundred thousand human beings, if not more than one million human beings in the FIRST HOUR OF CONFLICT. He has artillery in place on the NK side that is embedded into the mountains looking over Seoul. We do not have the ability to neutralize even most of it before it can be used, even if we strike first. Even if we "nuke" NK, we cannot nuke these installations without also nuking the capitol of SK.
Let me just say it again. #1 we cannot nuke our way out of millions of casualties in allied South Korea. We can't. #2 NK does NOT need to use nukes to kill over 1 million people in Seoul, South Korea, they have the conventional artillery in place in the mountains overlooking Seoul to absolutely devastate one of the largest cities in the world. Within the first few seconds of all out conflict many thousands of South Koreans would die, it would take no special direction from Kim Jung Un. We cannot root out theses artillery pieces that are buried in the mountains, very well fortified, quickly. For one, there are many installations. MANY! Second they are well protected and placed in heavy mountain terrain. #3 every couple seconds they launch another salvo into a densely populated city.
I'm pretty sure Un can set a human kind record for most people killed within an hour with non-nuclear weapons.
...And I haven't even mentioned that he also has nukes.