The Unification War
The 6 Nations Period ended with a lot of bangs. 8 years before this war was Humanity War 3 (where humanity replaces world due to this thing of humanity having gone to the stars, but WW1/2 get counted as HW1/2), in which the death toll was massive and the war was only stopped because of the ridiculous economic cost to all of the 6 nations (as it turns out that constantly replacing expensive warships and repairing combat suits is expensive) - which then led to a really crappy economy even up to and during the Unification War.
The Oriental Empire and United Commonwealth both experienced coups by generals from the previous war and then merged together into the United Human Nation, despite having the Scandi-East European Union between them, declaring that all humanity should be under a single banner. The Scandi-East European Union was the first to fall, as it had left almost all of its warships in disrepair to focus its resources on rebuilding the economy and was, as previously mentioned, sandwiched within the UHN.
Then it fell down to war with the US, Russian Federation and Indian Republics. At first it was done conquest style but over time the words "grinding" and "attrition" applied more and more, with the UHN being better at coordinating its resources (and having slightly more than its enemies) and so ultimately coming out victorious. Some argue that the war continued until the end of the UHN as, even after the final taking of the Russian Federation that's often considered the end of the war, the UHN never stopped taking individual planets that refused to acknowledge UHN overlordship.
The Conflict
Prelude
Some generals performing coups on their nations and being like "yo dawg let's get humanity united".
Deployment
Lots of experimental styles of warfare were done, but it was mostly all about trying not to be noticed by the enemy, shaving away at their numbers before being inevitably being noticed, and then it falling into total chaos where the side with more fire/manpower at that point usually came out on top in any battle.
Battlefield
Everywhere saw at least a little combat. Border areas were basically torn to shreds by plasma fire and explosives.
Conditions
Well it always starts with "okay we're not using utterly cruel but effective weapons and methods" but when both sides just wanna get it done with...
The Engagement
Ever seen snipers on motorbikes fighting each other from miles away while both the bikes and the guns have automatic systems and each sniper has like 20 robots fighting alongside them? Yeah because that happened and not that rarely either.
Outcome
All of humanity under a single nation and the merging of militaries containing people who hated one another.
Aftermath
PTSD-afflicted veterans and civilians-who-were-in-or-near-battles, economic ruin, starvation as the UHN insisted on flipping to capitalism immediately after the war rather than controlling resources to ensure that they were assigned to best suit the goal of economic repair rather than the goal of ensuring the financial success of specific individuals.
Historical Significance
Legacy
"YAY! Humanity is united! Oh, oppression? Systematically starving locations where people were overly rebellious to foster reliance on the government and break the will of any rebels? There was a successful coup a little under 100 years later, but tonnes of rebellions and attempted coups that failed before then?"
In Literature
Plenty of war stories, poems, art, etc.
Technological Advancement
While there wasn't too much progress in terms of technology, there was in terms of "wow those guns sure got bigger didn't they?".
Included under Conflict
Included Conflicts
Conflict Type
War
Battlefield Type
Space
Start Date
2203
Ending Date
2225
Conflict Result
The UHN won.
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Belligerents
Strength
Slightly less resources, personnel etc than their enemies but better coordinated and able to get a wider recruitment due to there being a cause behind it.
Casualties
Oh boy. Let's just say big numbers. Big numbers will do.
Objectives
Conquering the enemy 3 nations and bringing all (or at least like 90%) of humanity under its banner.
Strength
Beefy boi but very inefficient with resources and many of the tacticians ain't too great.
Casualties
Lots. Around 5% of the Russian Federation's population.
Objectives
Not getting conquered by the UHN
Strength
Beefy boi but very inefficient with resources and many of the tacticians ain't too great.
Casualties
Lots. Around 5% of the Russian Federation's population.
Objectives
Not getting conquered by the UHN
Strength
Hey, could you stop flinging people at your enemies without properly equipping them first? Lots of people, not enough gear.
Casualties
Unsurprisingly, Red Army style tactics will get you Red Army style results. Oh boi, is that 13% of your population dead?
Objectives
Not gettin' conquered.
Strength
Bruh, did the private companies you buy weapons from because for some reason you refuse to make more than 20% of your own weapons just secretly sell most of their weapons to the UHN because it was a higher bidder, a deliberate move by the UHN to starve you of weapons? Oh, and now you're only sending the soldiers you can fully equip but it ain't enough *surprised Pikachu face*
Casualties
2% of their population. Lucky buggers managed it by putting the smallest number of troops out.
Objectives
Not gettin' conquered.
I apologize for how poor a representation I have at the top of the conflict between the UHN and its enemies as Shaggy would obliterate One Punch Man, Thanos and Goku in a matter of milliseconds (and that's when he's taking his sweet time for sport while at just 0.1% of his power). The Unification War was, in truth, mostly a war of attrition with battles focused at "mercing dem fools on da other side" rather than "acquiring a nice strategic position" and so we should've picked fighters that have tonnes of defense and hardly any attack to better show this as wars of attrition (much like battles between metapods) are long, grueling and often utterly soul-destroying.