W is for World War III

It is an interesting fact that, in all of the known parallel Earths that have been found, only three ever actually had a World War III. Two of those were USSR vs USA nukefests, one taking place in 1965 and one taking place in 1987. In both, nearly all human life was wiped out and a nuclear winter covered the globe.

The really interesting World War III was on Earth 71, where nuclear weapons don't work. It was 2005 and, deprived of hot radioactive death, the major powers (USSR, USA, British Empire, China) released a wide variety of biological weapons upon each other. Most of these weapons did a great job of killing humans, but a few mutated and swapped DNA with others and before you know it, people stopped dying and started mutating. Nobody got superpowers or anything like that, but these mutant humans were pretty much immune to most diseases, including those biological weapons. In addition, no two humans looked exactly alike. That guy might have blue skin and two left arms while some other dude might have four working legs and three eyes. Everybody looked strange and different and none of them were very happy with the folks who had caused it.

So they killed every last "normal" human and they started society anew. With no racial or ethnic or political barriers (since all of that stuff was deemed "old ways"), the new societies got along remarkably well. Now, five generations later, there is pretty much peace on earth and good will towards everyone. If you've got to have a World War, it's nice to have one with a happy ending.