The UN has had the chance to intervene into a global war yet.  Chances are though that it will fall apart before a major war.  Take the five permanent members of the Security Council, China, France, Russia, the UK and the United States.  All the most likely people to go to war are the founding members, which means that either a) they won't ever got to war, or b) the UN will fall apart and have no possible means to stop a war.