[Cronislee]

WWIII hasnt happened because the USA met the USSR at every turn with the understanding "You nuke us, we'll nuke you."
Early 1990's, Executive Outcomes enters Angola and fight the Angola rebels who had taken over the nation, raping and killing and mutilating whoever they could find. 300 Executive Outcome soldiers take out over 10,000 Angolan UNITA rebels, and liberates much of Angola from the rebel regime.
UN enters- ousts Executive Outcomes and thereby banishes Mercenaries from entering African campaigns. UNITA forces re-enter Angol, and take over 500 UN peace keepers hostage.

1994- The Rwanda Genocide begines. UN Peacekeepers enter the scene, but result in little to no change in the situation. Executive Outcomes attempts to enter the scene towards the beginning of the Rwandan Genocide in an attempt to stop the Hutu army from killing anymore Tutsies. The UN refuses to allow EO into Rwanda. The Genocide continues for several months.

Thats just two examples of incompetance by the UN.

If anyone has prevented WWIII from happening it was the USA, not the UN.
Not many nations are willing to go toe to toe against the USA's arsenal.

Thats not even bringing up Darfur, The Georgia campaign, and a number of other events that have accord across the globe.