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The Fictional United Nations is a formal international diplomatic apparatus that mediates in disputes between nations. Its primary goal is usually to prevent wars, and it sometimes forms out of an Enemy Mine or Fire-Forged Friends situation, as was the case with the real-life United Nations.
The concept is similar to The Alliance, but can contain both good and evil members, which provides a story set there with a source of conflict. Also, alliances are often just a treaty or even an informal agreement making nations allies, whereas here there is a formal governing apparatus. However, unlike The Federation, the Fictional United Nations is not treated as a single state: Individual members are still sovereign nations and may take action unilaterally if they feel the need. It also tends to have no military of its own, and operations ordered by its (usually) ruling council are executed by the armies of member states.
This can fall practically anywhere on the scales of Idealism Versus Cynicism and Romanticism Versus Enlightenment. In some portrayals the Fictional United Nations is hamstrung by applications of Realpolitik and greed, with big, powerful countries essentially free to ignore it without much consequence and run riot over weaker polities. In other cases the author depicts the job of keeping the peace between nations as tricky but not impossible, with the participants more inclined towards enlightened self-interest and willing to work together for the greater good, allowing for Aesops and/or Crowning Moments of Heartwarming when peoples of many tribes and creeds come together and do something awesome.
The Fictional United Nations may enforce a set of Fictional Geneva Conventions. Compare The Alliance and The Federation, as well as United Space of America, which has a similar based-on-a-real-organization derivation. Also compare Multinational Team. See also United Nations Is a Superpower, where the Fictional United Nations has a lot more power than usual; the logical conclusion of this is One World Order.
Iron Sky has the United World Confederacy which consists mainly of bickering delegates representing the United Kingdom, the United States, Russia, India, Japan, China, Pakistan, Finland, and North Korea.
Street Fighter has the Allied Nations, who deploy Colonel Guile and his men to Shadaloo in order to stop Bison. However, when Bison manages to best them, the AN then order Guile to retreat, much to his chagrin.
The Austin Powers movies have the World Organization, which mainly listened to Dr. Evil's broadcasts and such.
The later books in the Old Man's War series partly revolve around the formation of an apparatus called the Conclave, intended to end the incessant territorial warfare between the thousands of races in the setting and distribute planets fairly. The Last Colony and Zoe's Tale deal with an attempt by humanity to sabotage it out of distrust.
The Thomas Dixon Jr book The Fall Of A Nation has a "Parliament of Nations" created when the United States does not intervene in World War I, and that conflict ends in a negotiated stalemate. (The book was written before the US entry into the war.) The delegates are the head of state of each member nation, and it's even more useless than the real-life League Of Nations.
The 21st-century timeline in Star Trek: Federation features an organization in the background called the New United Nations that apparently replaced the original. We never find out much detail because the Optimum destroy the entire organization in the course of their bid for world domination.
The name first came up in the first episode of Star Trek: The Next Generation. It made a human-rights declaration in 2036, and from the context it had collapsed by 2079.
The Stargate SG-1 episode "The Torment of Tantalus" speaks of the Alliance of Four Great Races between the Asgard, Ancients, Furlings, and Nox. Ernest Littlefield's notes called it a "United Nations of the stars," but as the Asgard are the only remaining members to interact with the cast on a regular basis little is known beyond that.
The eponymous station served as a United Nations of sorts, with ambassadors from the League of Non-Aligned Worlds, the Narn Regime, the Centauri Republic, the Minbari, the Earth Alliance, and the Vorlon Empire each having a vote on various diplomatic resolutions. Often (as with the Real Life UN) this council proved ineffective in some crucial matters, ultimately (by admission of Ivanova in the Season 3 opening monologue) failing in its mission for peace as the Shadow War expanded. It didn't help that one of the ambassadors (Kosh, the Vorlon) rarely even showed up to the meetings, despite that ostensibly being his only official reason to be there, or that the representatives would sometimes cast their vote in spite of their government's instructions (Londo, and Ivanova acting as Sinclair's proxy, both in the same episode).
It also didn't help that the League of Non-Aligned Worlds, consisting of dozens of races, only had one vote on the council, which is what eventually led to the League's dissolution.
The Interstellar Alliance, which formed toward the end of the series after the Earth Civil War, was probably a more effective version, as each member was mutually protected from aggression by the treaty binding all members, which was enforced by the organization's military force, the Rangers.
"Locksley Hall" by Alfred, Lord Tennyson correctly predicts aviation, aerial commerce, and aerial warfare, then predicts that the wars will be ended by "the Parliament of man, the Federation of the world".
In the X-Universe series the five core races (Argon, Boron, Paranids, Split, and Teladi) form a group called the Commonwealth, or more formally the Community of Planets. The Teladi spearheaded its formation in the course of their efforts to enrich themselves off of trade, which the rivalry between the Argon/Boron and Paranid/Split alliances made difficult. The Commonwealth members still fight occasionally but they also band together against common threats such as the Khaak.
The Citadel government is headed by a Council of appointed representatives of races that have demonstrated a commitment to the welfare of the entire alliance, both economically and militarily. Initially it's just the asari, turians, and salarians but after Mass Effect 1, humanity gains a seat (or in one ending, takes it over completely). The Council is somewhat of a benevolent dictatorship in that any decision they give must be abided by, and non-Council races can only make their case and hope the Council agrees with them. Unusually the Council can enforce its decisions fairly readily: though it doesn't have a formal military of its own it does have an office of elite covert operatives, the Spectres, who legally answer only to the Council. Larger problems still require member governments' troops, though.
The Systems Alliance, the de facto government of humanity, was originally founded as an effort of Earth's individual nations, as a means to organize the colonization of space (hence the name: systems as in "ones other than Sol"). At first, it had absolutely no power on Earth at all. It was only when the First Contact War started that they came into their own: while the various Earth nations bickered about what to do, the Alliance sent in its own "peacekeeping" forces to kick alien ass. This lead to the Alliance being given the power to conduct diplomacy and war with alien powers without consulting the old nations — and since this includes the terms of humanity's Council association (and later membership), they have basically all the power they could want.
UPEO, short for "Universal Peace Enforcement Organization", in Ace Combat 3: Electrosphere is an international peacekeeping entity in a 2040s Strangereal where the national governments have long been supplanted by a world-spanning Corporatocracy. They are the last relic of the national state era and are therefore incredibly unfit to handle serious Corporate Warfare that forms the premise of the game's plot.
In the Ace Combat Strangereal Universe, UPEO is actually an organization under the Neo United Nations (NUN), which is itself a successor to the earlier International United Nations (IUN), which is itself a successor to the Congress of Nations (based on the League of Nations). All probably fit this trope. They all, also, do very little in the plot of the games.
Sid Meiers Alpha Centauri has the Planetary Council, a UN-like organization that consists of all human factions (the two Progenitor factions in the Alien Crossfire expansion can't join) except those who have been eliminated or committed major atrocities. The Council can first be called by a faction that has obtained communication frequencies with all others. The first task is always to elect the Planetary Governor. A Governor can call the Council to make a single decision every 10 years, while any other faction can do it every 20 years. For most decisions, the factions have 1 vote each, although the Planetary Governor has veto power. When electing a Planetary Governor or a Supreme Leader (diplomatic victory; only possible in the expansion if no aliens remain), the number of votes each faction has depends on its population and secret projects. Before a vote, it's possible for backroom deals to take place, where one faction agrees to vote a certain way in exchange for something else. For example, the UN Charter is initially in force, meaning all atrocities result in harsh penalties from other factions. The Charter can be repealed (and reinstated later) by a vote in the Council, resulting in such actions being considered normal parts of warfare (except the use of Planet Busters).
In Civilization V, after a civilization meets all the others and researches the Printing Press tech, the World Congress is founded (you can change the name to whatever you want, though). Every civilization starts with 1 delegate, but the host civ receives an extra delegate. The number of delegates also depends on the civ's current tech era and whether the United Nations wonder has been built. From the Industrial era onward, each allied city-state also grants a delegate to the Congress. Delegates are also granted to a civ that builds the Forbidden Palace wonder and for placing spies as diplomats into other civs' capitals. Enacting a World Religion or World Ideology means that any civ that follows those tenets also grants 2 additional delegates. Each session, the host (and one other civ) can propose a new resolution. The next session, the civs vote on the two proposals by distributing their delegates into the "aye"/"nay" selections for each proposal. A delegate can't be used on both resolutions at the same time. When selecting a proposal, the info box will let you know which civs will be happy/angry over this. Naturally, this can seriously affect your diplomatic standing with the other civs. You can trade for votes using the diplomacy screen, but only with civs with which you have diplomats. A civ that has pledged to vote a certain way cannot go back on its word.
In Terra the creation of one of these is a long-term goal of the Resistance. As it stands two powerful factions are fighting over everybody else's heads and everyone else is getting thoroughly screwed.