Nuclear weapon, or the threat of having one, is a reason that Iran and North Korea as still not being invaded.
I would agree with you on North Korea. The only reason why they are still not being invaded is due to the fact that they are having nuclear weapons. But I am not sure about Iran. They are saying that they abandoned their nuclear weapons program. But who knows for sure?
Who said that North Korea fear? It seems to me that the Americans just deciding how to destroy the Kim regime. The Americans can turn all of North Korea into ruins in one day. While Kim has no chance to respond adequately to the United States.