Also saying that poor people can just not eat meat or seafood seems a bit distasteful; not that you might be wrong.
WW2 was quite dangerous in the sense, it put a very serious problem in front. Say you nuke a nation, the nation refuses to surrender. The issue is you need to keep nuking till you finish them off. That will not just finish off your enemies but a lot of planet along the way.
Moreover, the enemy has begun to employ a new and most cruel bomb, the power of which to do damage is, indeed, incalculable, taking the toll of many innocent lives. Should we continue to fight, not only would it result in an ultimate collapse and obliteration of the Japanese nation, but also it would lead to the total extinction of human civilization.
From: https://en.wikipedia.org/wiki/Surrender_of_Japan#Broadcast_o...
The key in the above statement is : "Should we continue to fight".
Notice how 'continuing to fight' is an option there. This sort of stuff is scary as it is.
You can't exactly defeat enemies by weapons if they refuse to give up. The only option then is continue, you then cause larger damage to the planet and biological species.
Things like deaths of mesoamerican civs due to destroying their environment http://news.nationalgeographic.com/news/2003/03/0313_030313_.... Or all of america after 2nd european invasion. Ever hear of the mississippi mound builders? I'd expect not since there civ was totally wiped out.
And civ bending doesn't mean everyone's dead. Just large enough population drop to break the institutions and social order supported by that population.
Famine and drought would be localized so we can use international trade to smooth them over, just like we currently do. For long term failures, there will always be other arable places to grow crops, they just might be fewer or more inconvenient than they currently are, and they'll change at an easy human scale speed. China has a 300,000,000 migrant worker population. That's all of the US population migrating every year!
We currently produce far more food than we need, so even losing a big part of production capacity will still leave us with enough to eat, but perhaps with less meat or seafood for poor people.