The UN was founded after the nuclear bombs were dropped.

Yes, it did. It's not like only one thing is responsible for all geopolitical happenings. The UN acts like a pressure-release valve, allowing countries a forum to air their grievances. It helps reduce the tension.

Think of all the things that the security council has vetoed. In the old days, those things would have had a decent chance of being tried anyway, which in turn may have had the event escalate to something truly horrible as one of the great powers tries to save face. Things like the Cuban Missile Crisis would have happened again and again.