All the theories I read have to have a lot of things line up for it to actually start. However know one has been talking about who would win and mainly I think its because there can be no winners this time around with all the new weapons and bomb tech most countries now own.
My guess is fairly general but something along the following lines:
Climate change fueled freak weather occurrence/years of such lead to massive food insecurity (or massive influx of refugees from a nearby country suffering from the above) in a poor but nuclear armed nation (think Pakistan) leading them to freak out and elect someone crazy (like America did with trump when they went from being the richest country in the world to still the richest country in the world.
Results: crazy threats, escalations, someone launches a pre-emptive attack or there’s a coup, nuclear weapons are launched in the havoc and then things really spiral.
So Fallout?