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?
It's fact that their leader Kim is only decision maker in whole country.
There is no democracy of any kind of government control there.
Another fact is that Kim is crazy and mentally unstable.
So, do you understand now why all neighbors around North Korea, South Korea, Japan... fear Kim and his nuclear bombs.
They don't want to destroy North Korea but to destroy their nuclear bombs.
Seoul, capitol of South Korea, is only a half hour drive from the border with the north Korea.
If anything happen, millions of people there will be killed.
Kim is threat to all international community.
He don't listen numerous resolutions from UN and continue his nuclear program.
Other countries, with nuclear weapons, USA, Russia, UK etc, are stable and with strong government control, so they are not threat to others because of their nuclear weapons.
They will never use nuclear bombs against any other country.
Kim is different story.