When the Iron Curtain came down it was quickly discovered that the USSR never accepted the MAD, Mutually Assured Destruction, doctrine that had guided the U.S. nuclear strategy for 35 years.
The MAD doctrine focused on the use of a second strike capability. If the USSR or China used a first strike to attempt destruction of the U.S. then a second retaliatory strike had to be a certainty. That would make sure a first strike never occurred.
The USSR was found to ignore MAD and assumed that a first and second strike would deplete both arsenals and leave the USSR in a survival state because of good civil defense ( Moscow subways are very deep underground) and able to fight a traditional winning war.
At this point we have a different scenario. We have nuclear Israel vs Iran and nuclear North Korea against Japan.
These are different situations than MAD or USSR ‘survival’. Israel is two major targets with 80% of its population and Japan is also two targets, both very large but about 70% of the population. Iran and North Korea each have one target about 80% of their population.
Since neither Israel nor Japan will start a nuclear exchange I argue that they are relatively immune to a single enemy that could be considered a ‘mad man’.
Both Israel and Japan have a narrow target area, in the sky, relative to an incoming missile. Therefore both countries can afford a massive missile defense shield with dozens of defensive missiles. This is true as long as the risky enemy is known and identifiable.
From a survival point of view I think both Israel and Japan can deal with their single enemy being armed for many decades into the future. If those enemies obtain nuclear armed submarines, the situation would change.
However despite this analysis, it is my opinion that even one nuclear bomb exploded by an enemy will bring the entire modern world to an end. The level of freedom necessary in the U.S. and most of the world for modern commerce to continue would be destroyed by the fear engendered by one nuclear exchange.
MAD may be gone, but the nuclear bomb era is more dangerous than ever.