North Korea is under surveilance of Japan, South Korea and Taiwan more than anybody. Japan and the others worry everyday about North Korea. Therefore they have ways to deal with North Korea no worries.
North Korea was lucky when nuclear weapon was uncheck and uncontrolled. Everybody knows how could build it, not so hard. So, North Korea has got away with it. But these days nuclear weapon is a threat to human kind. It could kill so many so fast.
Nuclear bombed Japan was only very crude bomb. If it was not for the bombs extremely strong Japanese's marines and army would had begun a war on mainland U.S just like when Japan invaded China a country 10 times its size. It was a time everybody was fighting each other for power. Killing another human being is just for fun. That is why it was World War II.
For the first time in world's history, we human being could enjoy peace. Don't know how long it is going to last. But we like it, we could live to 100 years old and die of old age. Like many asian countries they went through years of fighting they are tired, sick of fighting for stupid reasons. Many did not live to see 30th birthday.