So the peace of the 19th century saw a period of relative peace (i.e. without great power conflict) that lasted from the congress of Vienna until world war one.

So the peace of the 19th century saw a period of relative peace (i.e. without great power conflict) that lasted from the congress of Vienna until world war one. There were no nukes, but rather a period which saw the spread of democracy across Europe (but also European colonialism). This was a 99 year long period. The Cold War lasted for less then 50. What makes you so certain that full scale warfare is so unavoidable if not for nukes? Secondly, the cold war is over. What I alluded to was that the world would get rid of nukes in the period after the election of Gorbachev, but before the election of Bush. At this point the cold war is pretty much over (if not completely over; and it is certainly over now). And in this post-cold war era, Russia—a nuclear power—invades Ukraine. Why not get rid of nukes now? And finally, I believe the Earth can be whichever shape an author wants it to be in fiction. Alternative history is fiction, so in your alternative timeline of no nukes causing WW3, the earth may as well be a donut.