I fear that a 'decent world war' nowadays may include nukes, so damage to the environment will be great.
As an example, the dam that was destroyed in Ukraine and flooded a 'small area' (considering the planet-scale)(https://www.cbc.ca/news/world/kakhova-dam-ukraine-russia-1.6...).
Which (your post and my thinking of a response) reminded me of the series: Aftermath - Population Zero (https://www.imdb.com/title/tt1264068/); I remember watching some series back in the day.. I think it's time to watch this again.
I'm slowly starting to realise that nukes are very unlikely to be involved simply because the selfish shits who start and run the wars know they personally won't survive if they start the nuke match.
Which (your post and my thinking of a response) reminded me of the series: Aftermath - Population Zero (https://www.imdb.com/title/tt1264068/); I remember watching some series back in the day.. I think it's time to watch this again.
EDIT: I found it on YT: https://www.youtube.com/watch?v=l11zPNb-MFg