Humanity dwindled to a population that could realistically be controlled by a single government and that single government simply remained in power as the population grew back. Yes there are rebels and sub-groups/edge-case sovereignties but 98% of the planet is still entirely controlled by a single government.

A lot of war and eventually the First Nation got the jump drive (the United Nations of Europe) and that kinda ended the fight when they had a stick so big. For them nuclear war became not a problem but an annoyance. Later on the UN was reorganized to allow them and from there they had about 200 years or so of prosperity exploring the universe and totally not settling old grudges. At least until the earthless incident occurred.

Ouroboros:

I actually have two different ways it's happened, thanks to two different Earths being involved.

On Terah, it essentially turns out that somebody "won"; they developed a technology that they used to subjugate everyone else and created a worldwide empire. While the vassal states did eventually stand up again and resist being openly oppressed, existing for a time as a unified government sort of ossified the concept, and "free states" became the exception, not the rule.

On Old Earth, the unified government... still kinda isn't. Rather, it's a commission put together by various world leaders to handle a specific threat (Terah) and set policy accordingly. Most of the world has buy-in, as most of the world doesn't want to be invaded, and authority creep is slowly turning the alliance into a form of worldwide government (but democratic!).

It’s a semi-unified shadow government, concerning only the supernatural beings of the world.

The Treaty of Claw, Wand, and Fang was signed by representatives of each “class” of Xenodem (supernatural beings), brought together by Toni Squomito after he missed the telephone in use for the first time. He recognized immediately that such mass communication would make it much more difficult for the Xenodem to hide, especially from the Spanish Inquisition.

The Court of Cats, previously only concerned with the traditions and safety of shapeshifters, was named the head of the new government, and restructured to represent each class of Xenodem. Through them, laws were decided to help hide them from mankind.

The Court has no permanent seat, but convenes at least once each year to hear trials and discuss new laws or changes needed. These sessions last anywhere from two weeks to six months. Emergency sessions have also been called when needed, such as the emergency session of 1992, when the court convened to try the Mad Sorcerer, Ark, for his blatant act of magic that nearly revealed them all to the modern world.

They found alien tech in the solar system, and after that grassroots movement, supported by elder statesmen, retired military leadership, public servants and included youth movements put it through

They control most of the world's magic oil for the world's flying trade ships. Of course, this encourages piracy but that's the point in my story.

Violence, a lot of violence

The Pax Terra was create through an incredibly violent war, that is historically attributed to the "injustice of diplomacy", as the exclusive contact of some extraterrestrial forces with only a few governments was called.

This contact allowed these few selected governments to accelerate at absolute breakneck speeds, but also refused an open sharing of these technologies (as the extraterrestrials, the Siccom, expected the humans would do).

When the Siccom returned decades later to install an ID Gate in earths orbit, they found humanity reduced to 50% of their population, entire regions irradiated and pulverized, and an incredible war still raging on that had devolved from one of "justice" to one of annihilation.

It was ended by Siccom intervention, who realized they had to choose a side of what they originally thought were merely organisational factions. The Pax Terra was founded as safe havens, named IORS, which were also ultimately forced to arm themselves in order to protect those havens. Identified as the most peaceful section of humanity, they were given war ending weapons, and most importantly, access to the wider realm of interstellar and intergalactic diplomatic relationships.

All humans that exist nowadays, descend from those having organised and/or sought shelter in an IORS refugee city. The name Pax Terra - World Peace - stems from Latin, and was voted for by the assembly of surviving cities.

Haven’t gotten to that part yet, so idk

In one world I made up that is a post nuclear war America where because of a lot of political turmoil resulting in a 2nd civil war where one of the sides detonated a nuke in one of the opposing side cities. Amidst this, some of them panicked about foreign invaders and launched nukes on other countries.

Afterward when everything was calming down, everyone just decided to cut off America and leave it for dead (including the president and other important people hiding in bunkers), so the world was left to think about what just happened. One thing led to another as people were using the implosion in America that right-leaning political beliefs always lead to issues as evident by past behaviors, so... in a way, the whole world plunged into their own civil wars.

Eventually though, after some back and forth fighting, lot of countries either ended up joining together to form a new United Nations and started federalizing regions (like the region of Europe becoming the federalized country of Europe). Governments like the CCCP in China was thrown away in favor of the New Democratic Party of China, Russia would become a socialist country like the ones in Scandinavia, etc.

Only problem is that when establishing a lot of left-leaning countries, they in turn started getting rid and also getting stricter on right-leaning beliefs out of fear that more conflicts will happen. So... the whole world apart from America being a snowy wasteland became authoritarian leftists.

Point with the story is usually about just how insane political unrest can lead to

https://preview.redd.it/wvmi6aryzr8g1.jpeg?width=393&format=pjpg&auto=webp&s=6f80e41680b963f34aa00536d71c4ceb2d080a2c

There are only a few thousand people and a whole lot of monsters.

The most simple way of unification imaginable. They just found a non-negotiable common enemy race. either fight that enemy or face total humanity extinction.

Mild nuclear war -> surviving nations agreeing not to destroy humanity (similar to the setting of lancer but less extreme)

World Government is just a puppet, the real people that control the world is Enterprise Alliance, a loose union formed with several super corporations, they controlled every part of earth surface. The only part they can't control is automatic generator cluster that occupied the whole moon surface, and the war-zone, a anarchy colony wreckage at the Lagrange points

Earth isn't the centre of human space anymore. There are three major human nations outside of Earth, a minor one, and Earth is just in the centre of it all. At some point the need for nations on Earth kind of dissolved as it basically became a tourist trap planet, the "cradle of humanity" and little more. It's run by the United Earth Government, which is itself barely more than an administrative body as the three major players effectively control the overall interests and defense of Earth itself.

Earth more or less united out of space travel necessity, since FTL travel was still in development, another reason is due to a colony ship, with millions of colonists that was supposed to go to Trappist-1e just mysteriously disappeared, earth ended up uniting into the Earth Protectorate, which at the time, was very fragile, it eventually became stable slowly, the government of the Earth protectorate shifted into a sort of directorate government, humanity advanced a lot more and developed FTL travel through the discovery of an extremely energy dense gas called Tesserene gas, which could be used renewably in a reactor, this is how the Earth Protectorate united.

I kinda glossed over it, but essentially, the hyperdrive. Suddenly, people that didn't mesh well could leave and go elsewhere. It also reignited the wonder and exploration vibe of the Age of Discovery and the original space race and the cooperation from it. So the United Nations Outer Space Council (I added the O to avoid just the UNSC) was formed to handle the use of space for everyone and it kind of...ballooned from there. Nations working together, the stars being treated like the seas.

I didn't focus as lot on it, because the series is 2600 years in the future.

I don't have one atm but one of my nations wants to achieve this as part of their "Manifest Destiny". Essentially their goal is for all countries to function as states that answer to the capital.

The opposing side takes a EU like approach where all nations are united but independent.

Both sides are currently in a stale mate war and both are fighting through small proxy wars because both sides don't want to fight because doing so would result in a nuclear armageddon.

In my world, it's either achieved through Hive Mind Superpower or a super-technology.

For Hive Mind Organism, there is Slime, soulmancer (mastery of soul), Elder Lich, and others.

For super-technology society, it's usually achieved by wars, supremacy, and homogenization.

Fear. Aliens accidentally showed up(the Solar Systems Union) and earth kinda panicked their way into joining.

Genocides. The only way it can be done.

The most powerful and important entities cutting attachments to their homes and gathering between them to mange a system that removed the power from most positions forcing every nation leader to be a servent to their people Rather than king and making harsher punishment to spies and traitors with the ability to plea for life sentence for some and if good behavior even have it as house arrest

Slavery returned for a while but eventually died out when the last slave was freed 200 years after the end of the pyre hunt (some called it the unifying war but the lords refused that title)

Some rights are gone some where added certain places had their social structure completely changed and the the difference between poor and rich is less

There's also a limit to how much unmoving money a person can have and to what can they sell or market

After the the new resource distribution system came to be people no longer slept unsheltered or hungry and while after that theft and bribery had their punishments elevated

Very few "rich" people remain and the lords all have clinical depression and have forgone ever hope, desire and chance for happiness in favor fulfilling their duties though sometimes they are allowed meetings which they greatly enjoy

There's usually some cataclysmic event or the threat of one forcing everyone to unite under one government- war, economic collapse, climate, aliens etc

I don't think that's super realistic, personally, so anything I've explored has a 'world government' but really they're the face of a still divided Earth. Think how the Systems Alliance in Mass Effect works.

My current project "Pendulum's Tear" envisions the later decades of our century, starting in the current day, and how humanity will get so centralised that a world government becomes the status quo (even after the first fails).

The way the first comes around is by WW3 (conventional warfare only), which I set to start in 2026 (yes), and the following chaos. One major factor is that the power blocs mostly opposed to a world government were wiped out by the war: The religious conservatism of the Islamic world, the nationalist conservatism of Russia, and the religious conservatism of the USA.

The new order would be mainly manipulated and taken advantage of by China, with other great powers on the rise and competing for influence. The UN would be dissolved and replaced by a more potent organisation equipped with strong executive abilties, called the "International Constructive Federation". Most of the world population would harbour great anger towards their governments and consider the nation state an obsolete concept, resulting in the central government winning the propaganda war about humanity's course. Smaller conflicts, instability and natural desasters would speed up the unification process, which would be completed by 2036, and this world government would go on to remain stable for 25 years, until dissolving in a Global Civil War.

The allied cults control most of the planet, aside from holdouts in what is now East Russia and central north America.

Powerful religious authorities guide policy from the top, putting out calls to action that local powers, religious and secular, react to, competing for prestige and recognition. The old secular governments remain, doing all the day to day governance of their territory, but having no real voice on what we now know as "foreign policy".

A unified government would work if it wasn't a human government, or if the population had dwindled to maybe a couple hundred million people.

I don't believe they'd work, otherwise.

I think it depends on the planet.

On Earth, explicitly, an alternative history makes a lot of sense IMO.

I personally would build on the bones of real world history. Events like:

• 9/7/1923 - International Crime Police Commission (ICPC) established

• 10/24/1945 - UN Charter ratified by the Security Council

• 10/30/1947 - General Agreement on Tariffs and Trade (GATT) signed

• 1956 - ICPC becomes Interpol

• 1/1/1995 - GATT is replaced by the WTO.

All show a move toward a more global perspective during the 20th century.

Add in a couple of interesting revisions like:

"During the first summit of its kind, in 1999 an intergovernmental panel of experts proposed a new framework for tackling problems on a global scale.

This Foundation, rolling together the established infrastructure of the UN, the WTO and INTERPOL as a planet wide Governing, Trade regulating, and Law Enforcement body eventually became the de factor deciding force for all planetary operations. Over time, individual governments became increasingly beholden to this Ministry for Earth Global Affairs. By 2020, individual governments didn't dare to defy resolutions passed by MEGA, for fear of absolute global sanctions and being completely expelled from all foreign markets."

And you've got yourself the beginnings of a world government.

Hope the example helps.

Alien invasion forced the unification of humanity into a temporary government. Once they pushed back the invasion, they used their new technologically-superior WMDs to immediately wage war on themselves, making Earth uninhabitable. To avoid getting swallowed up by other polities, a new human government was created. 

Technically my universe has a failed "United Earth Government" that ends up being the impetus for a semi-successful (though corrupt) united human government.

lol no.

they expanded to the stars, but every settlement quickly became their own city-state.

eventually the nations on Earth just kind of evaporated, as more and more people bought starships or stations, and found planets to colonize.

nowadays, earth permanent population is around 1 billion. (mars is close to 350 billion currently, moon has 56 billion, venus has 120 billion.) Earth's population is divided into several dozen minor countries, which reside in arcologies. most of the planet is owned by the Department of the Exterior, which is sort of park rangers.

There is a "one government" but they mostly just serve to keep all the planets and cities and stations and countries from infighting too much, and to collect taxes and build armadas for defense. sort of similar to how the imperium handles its worlds, so long at the tithes (taxes) flow, and corruption is low, then they dont care all that much.

Though there are a set of foundational rules that every planet has to follow. sort of like how the US has a set of federal laws, and state laws. everyone has to follow the federal laws, but beyond those broad laws the states handle things themselves.

Humanity discovered the threat of aliens in a slow, piecemeal manner, with bits and pieces of extraterrestrial technology showing up on Earth and the moon. To reverse-engineer the tech, they needed the best scientists, mathematicians, and technicians to combine their brain-power...and to keep an eye on each other. As such, the fruits of their labor were slowly disseminated between the nations, new agricultural, industrial, and military opportunities were opened up, from powered suits to vertical farms to zero-gravity manufacturing to orbital solar panels. Many of these required enormous capital up-front and exotic materials from around the planet so corporations, nations, and the UN became partners in opening up the space economy, each contributing something vital to making it work. Corporations provided capital and intel, the UN a common legal and economic model, and the nations raw materials and labor. Over time, the bonds forged across the planet and between space and earth became stronger and stronger until the cost of breaking them proved greater than national prestige; any nation that wanted in needed to agree to a set of conditions to ensure all had a fair share of the economy. Those that refused were denied access to these new goods and services and were left to rot on Earth, while those that violated the conditions were slammed with ruinous sanctions or even military invasion.

Emperor Pentecost got fed up with the idea of governments half way through World War 3 and decided to conquer the world himself and run things the right way.

Lore wise he's the first and only human mortal to fully ascend into a higher being. Other humans have magic and superpowers, but he's fully built different.