The United Nations (UN for short) was a coalition of Earth nations.
In the late 20th century, the UN put together the United Nations Intelligence Taskforce (later the UNified Intelligence Taskforce) to defend the Earth against threats such as the Great Intelligence. UNIT Central Control was placed in Geneva. (TV: The Invasion et al) This was done after a direct appeal by Alistair Gordon Lethbridge-Stewart. (PROSE: Who Killed Kennedy) UNIT's international status would allow it to go over the head of British military figures, (TV: The Invasion) but this also meant the authorities didn't trust it. (TV: The Ambassadors of Death)
General George Carrington advised the UN Security Council to coordinate a global nuclear assault on the Ambassadors' spaceship. The Council could not agree on how to respond, leaving Carrington frustrated. (TV: The Ambassadors of Death)
On 1 January 1990, humanity subconsciously realised that there had been temporal discrepancies in the last two decades. Amara Essy, president of the United Nations Security Council, attended a secret briefing with Brigadiers Alistair Gordon Lethbridge-Stewart and Charles Crichton in Geneva to establish a dating protocol for the 1970s and 1980s. (PROSE: The Enfolded Time)
As late as March 2006, the Ninth Doctor identified UNIT as the "United Nations Intelligence Taskforce". (TV: Aliens of London) As late as December that year, British Prime Minister Harriet Jones noted that the UN remained unaware of the Torchwood Institute, which she herself was not supposed to know of. (TV: The Christmas Invasion)
Following the Battle of Canary Wharf, Jack Harkness claimed to Gwen Cooper that the Torchwood Institute was "beyond" the United Nations, clarifying that he could not allow any one power to acquire the alien technology Torchwood collected. (TV: Everything Changes)
By the 2009 Sontaran invasion of Earth, UNIT themselves identified as the "Unified Intelligence Taskforce. However, their association remained, and due to a sharp increase in alien incidents, the UN increased funding for UNIT under a policy of "homeworld security". (TV: The Sontaran Stratagem) During the ATMOS crisis, the United Nations issued a worldwide directive advising urban populations to stay indoors and those in rural areas to stay away from all major cities. (TV: The Poison Sky)
In the immediate aftermath of the Earth's relocation to the Medusa Cascade, the United Nations issued an edict asking the citizens of the world not to panic. Following the subsequent Dalek invasion which saw UNIT massacred, the Commander General of the United Nations broadcast a message announcing Earth's surrender to the Dalek Fleet. (TV: The Stolen Earth)
During the Miracle Day phenomenon in 2011, which rendered humanity immortal yet still susceptible to injury or disease, China made a threat to the United Nations that they would undervalue the currency down to fifty percent. (TV: Rendition) PhiCorp established overflow camps for patients, issuing a strategy to the United Nations. (TV: Escape to LA) Soon after, the UN officially sanctioned the category system, which they called a radical new start for the health of humanity. Jack Harkness observed that this gave them the power to define both life and death. (TV: The Categories of Life) Eventually, China withdrew from the United Nations and closed their borders. (TV: The Gathering)
In 2017, the United Nations summoned the Doctor (as President of Earth), Bill Potts, and Nardole to Turmezistan to investigate the sudden appearance of a pyramid in a disputed area between American, Chinese and Russian troops. (TV: The Pyramid at the End of the World)
Between 2086 to 2088, Earth fought the Thousand Day War under the flag of the United Nations. The UN Third Tactical Response Brigade under Yembe Lethbridge-Stewart were one of the UN forces. (PROSE: Transit)
In the year 4000, Marc Cory, Gordon Lowery, and Jeff Garvey scout Kembel, the most dangerous planet in the universe, seeking to learn of the Daleks' presence and plans. Their ship is from the United Nations Deep Space Force Group 1.[source needed]
In the Unbound Universe, the Master, trapped on Earth without his TARDIS, found work with the United Nations. After the failure of the World Peace Conference however, he defected to China. (AUDIO: Sympathy for the Devil) By one account, the Doctor, aided by N-Space's Sixth Doctor and Evelyn Smythe, later saved the United Nations from "an annoying man" who tried to unleash Krynoid pods. (AUDIO: The 100 Days of the Doctor)
Behind the scenes
After the broadcast of Aliens of London, it was announced that the United Nations were no longer happy to be directly associated with UNIT. The term "UN" could, however, be used as long as it wasn't stated what it stood for. Later, UNIT was shown to stand for Unified Intelligence Taskforce, with the United Nations being mentioned as providing the organisation with unlimited funding.