Impossible to prove hypothesis: nuclear weapons prevented a world war.
Facing the facts about nuclear weapons means owning the good (probably prevented wars) and the bad (at the very least there were severe environmental and economic consequences).
The Cold War was the best possible outcome. The alternatives (super power hot war, nuclear armageddon) did not happen. Nuclear weapon potential was a large part of that, as was direct lines of communication. Unfortunately because the later is no longer a thing, and our president declined to extend the existing agreement (a problematic pattern), we now again face proliferation of nuclear weapons.
This is not mathematical logic. Of course you can't prove the causality of anything in history. The only way to do that would be to invent a time machine, delete the nukes and watch things play out.
There are direct quotes from Nixon and Reagan that claims this to be true. That is enough evidence for me.
Some people believe WW2 started many years earlier, but most historians don't put the start date at the conflicts going on before the war became intercontinental with sides working together on a global scale.
War has still been going on since 1945, much earlier if not always, so akshually WW2 never ended or is just how it's always been? (mostly in places that are not "first world" / The West)
There's a simple and very logical definition of what a "world war" is - it's when international law completely breaks down, and the only way to get back to a semblance of order in international relations is to have the victors of the ensuing shitstorm impose a new set of rules in some sort of grand finale pact.
1945 was clearly that. 1920 with the League of Nations attempted to be that, but failed.
Whatever happens after the current turbulence we can be sure that there won't be a UN and NATO and a World Bank at the end.
P.S. The first modern world war was the Thirty Year's War.
Because the UN, NATO, etc., are clearly not working now. They're not gonna be fixed to work as designed because that's quite impossible in today's world.
That's been a posited theory for decades - it's not just some HN edgelord thing. There are some obvious problems with it of course and the P5 have instead taken a maximalist attitude towards keeping the genie in the bottle (notwithstanding Atoms for Peace and accommodating India/Pakistan when it became inevitable).
Prevented a world war for 80+ years.