1.Israel stupidly keeps attacking everyone and US weapons supply runs dry. They get destroyed by their victims retaliation.
2.China actually decides to do something.
3.Russia does that Simpsons bit where robot Lenin breaks out of his grave. But says "Haha joke!" And continues being shitty and capitalist.
4.Ukraine does something stupid and Nazi-coded to get attention from US senpai.
5.The US accidentally nukes itself.
Not debunked, but it's all theoretical and controversial, as I say nobody really knows. Many still reasonably suggest there'd be mass extinctions from a "minor" nuclear war. Some disagree.
Basically everyone agrees there'd be some form of globally damaging fallout. Whatever the case, I'm not that eager to find out, personally.