Checkout the super relevant movie Threads. Shows the horrors of nuclear holocaust in excruciating detail. It takes place in the UK but the situation would be the same anywhere nukes hit: complete and utter devastation for all.

If any nuclear attack would be met with a counter attack using nukes, the whole world would suffer from the fallout that wouldn’t be contained only within the national boundaries of the countries that get nuked.

https://en.wikipedia.org/wiki/Nuclear_holocaust