There hasn't been a day without a shooting war since WWII, the difference is they are just proxy wars and not between the Great Powers because of htisCompeting, conflicting bilateral alliances caused WW1, the resolution of which was a major cause of WW2.
Since WW2 strong, multilateral alliances have kept international shooting wars to a historic low.
The UN has nothing to do with it. And speaking of competing alliances, what do you think NATO and the Warsaw Pact were?