Uhm. No.

and

No, you wouldnt.

Ok, I guess I will explain these nos real quick -

ww2 happened, essentially because the French were insistent on such severe treatment of the germans after ww1 that their economy never recovered, and eventually the germans got so bitter as to hand power to a creature like Hitler.

WW3 DIDNT happen, because nuclear weapons were in play. Weighing the effects of some organization like the UN, and the concept that ww3 would likely bring about the near-extinction of our species side by side... I find it difficult to fathom how the UN could have played any part.

If you think "world opinion", or "world governing bodies" had jack, or shit to do with.... say- Kennedy or Chruszchev's military/nuclear decisions for example, then I have some fine real-estate in Florida I would love to sell you.

Saying "then along came the UN and, BAM - no WW3 " is akin to saying " wow I saw Brittney Spears on TV, and BAM! - my constipation cleared up".

Maybe, there was somehow through some complicated process connecting the two.. but in the example given above, the subject had also downed several habenero peppers, and a bag of black licorice. (peppers metaphorically representing the A-bombs here)