What will make things really bad, but for many less people.

Confirming the GP's argument, that Nature wins in the end. Winning is easier when you are not fighting anything.

Keep in mind that the last world war resulted in nuclear attacks. I don't think we have any guarantee that "mutually assured destruction" will be enough to save us if there is another truly global war between the remaining super/nuclear powers.

And now we also have stuff like "depleted" uranium rounds which have pretty serious consequences on future generations.