The United Nations were an organization, disbanded due to the start of the Resource Wars.
In the Fallout universe in May 2052, the United Nations began to collapse, due to the start of the Resource Wars following the European Commonwealth's military intervention in the Middle East. The oil-rich states of the Middle East had raised crude oil prices to near confiscatory levels in light of the growing global petroleum shortage. Facing an economic disaster because of European nations' heavy reliance on imported Middle Eastern oil, the European Commonwealth had invaded the Middle East to force its states to lower oil prices or to seize their remaining reserves of petroleum. The United Nations actively tried to restore the peace with nothing to show for their efforts. As natural resource pressures grew and the United Nations' impotence to maintain a stable international order became clear, many member nations withdrew from the organization, prepared to use military force to defend what was theirs or seize what was somebody else's. The United Nations General Assembly finally disbanded the organization on July 27, 2052 after the majority of its member-states had withdrawn from it. Its last Secretary-General was a Japanese diplomat, Sakugama Okiri. It's former headquarters in New York were then sold to a major toy retailer to be transformed into a superstore.
For more information on the organization's real world counterpart, see United Nations on Wikipedia.