Thank you!
This also gives people a chance to do some wanton destruction in the past. For example we can say that many of the largest cities were nuked during WW3. This helps to make this "collapsed world order" thing going as places that were claimed to be the center of the world would cease to exist.

I also wonder how would this change urban development and the citizens' thoughts about metropolises in general. I mean some nut can detonate a thermonuclear weapon and destroy a good portion of the city. Having a single big city with plenty of people is such an inviting target. This may even prohibit centralization attempts out of fear.

Conversely the use of nukes in population centers is going to be one of the worst war crimes to know in this setting. So an nation which cares about its international relations would avoid that at all costs. So ironically the chances of anyone doing this would be reduced considerably. Although there's a posibility that even terrorists can obtain pure fusion nukes which is indeed a scary thought.