The Split of the Confederacy of Borealis was a series of events which lasted on the same timeframe as the War of the Decamillenial Transition within Human Space. These include many political maneuvers, declarations, and a large civil war. They eventually culminated in the Treaty of Tuuze which caused the previously-massive Confederacy of Borealis to lose the majority of its influence, power, and territory alongside the rise of massive numbers of smaller human (or associated species)-controlled polities throughout Milky Way and 70 other galaxies.
The Confederacy had been the dominant organization for human civilization since the disbanding of the URSS. Initially, the Confederacy acts as a loosely binding organization, similar to the United Nations on old Earth. However, as time passes and the scope of humanity grows, events slowly made way to the centralization of human territories. This was especially true after facing large, powerful enemies in the First and Second Galactic Wars. By the transition to the Terran year 10000 CE, most if not all human polities had made way for the Confederacy in terms of external relations, military force, and significant portions of administration. Many were eventually absorbed entirely into the hierarchy of the central government especially those within 10,000 light-years of Sol. Some systems however retain large degrees of their autonomy.
This had worked well in the early periods until the interactions with massive powers such as the Xcathlia and others came into the equation. Many perceived that the elites of the Confederacy were too accommodating towards the Xcathlia, especially with the privileged access of the Intergalactic Confederacy (a Xcathli-founded, and essentially Xcathli-led organization) to Space Station Alpha. Nevertheless these sentiments remained mere sentiments until the ever-growing front lines of each powers began to approach each other within the massive room of the supercluster. Occasional conflicts had by then began to occur between the dependents or protectorates of many powers, heating up relations and firing the furnace for separatism at the same time.
The historically vital Battle of Koosi Paglaus was a major question to the Confederacy Senate. For several years after war erupted between the two major superclusteric powers, the floor of the Confederate Senate was put in an endless political debate, and countless diplomats from multiple states attempted to persuade the Confederacy into joining their side in the war. Eventually however, the Confederacy joined the war as an ally of the Xcathli, to great dismay of many systems even within the core territories of the Confederacy who had wished for the government to stay out from such an unrelated war.
See also: War of the Decamillenial Transition
As one of the largest participants in the war, the Confederacy also suffered from one of the largest casualties in the war. While humans did gain some territory in areas such as the Large Magellanic Cloud (against the Ysau who had managed to hold a large portion of the satellite galaxy), it was not perceived as compensatory for the populace. When the Confederacy implemented a draft to maximize their fighting power, it was met with extreme dismay in outer regions and territories. Some local governments actively prevented drafts of their population through many methods.
With the massive deaths and losses arising from Operation Heavenly Descent, the separatist and rebel movements all gained significant ground especially considering that the attack had failed to achieve its objectives. Separatist movements won many local elections and significant portions of the Confederate military began to desert in order to return to their home planets which are slowly leaving the Confederacy.
The tipping point was the Aaedepa Incident where 30,000 Confederate military personnel were killed by the Xcathli in a (massive) friendly fire accident. Days after the incident, many of the newly-formed militia groups in outer planets and systems seized control of Confederate military installations, factories, buildings, etc. Central government officials suffered fates ranging from near-normal treatment to public executions (in the most extreme cases). Initial sparks over the first weeks resulted in about 25 million inhabited systems (out of about a billion) in the Milky Way alone declaring their separation from the Confederacy,
While the Articles of the Confederacy had allowed systems to cede at any time, this movement took the Confederacy by surprise. An emergency meeting was held, resulting in approval for emergency laws which allowed the Confederate military to engage secessionists and separatists as "threats to humanity" i.e. enemies. This move proved to be a massive blunder since it swung even previously-loyal systems to secede on their own, perceiving that the Confederate government had prioritized alliance with aliens which had not served them well over other humans and its own law codes. In the end, over 50% of systems in Milky Way and nearly all extragalactic territories of the Confederacy left the Confederacy.
These systems consist a large part of the CoH military and officer staff. In general, lower-ranking personnel defected or deserted into their own systems although this is less prevalent amongst higher-ranking officers. A brief attempt of the Confederacy to take some rebellious systems were easily repulsed due to near-zero morale of their personnel (very few human personnel were willing to carpet bomb a human planet, for instance) and dwindling numbers. This forced CoH to sign ceasefires with parties in the WotDT, leaving the Xcathli to fend for themselves.
Slowly, the Civil War slowed down, with many Earth politicians beginning to accept the situation of territorial losses as a fait accompli. However, some of the secessionists wanted to entirely dissolve the Confederacy. A loose, powerful group among the seceding systems, known as the Commonwealth of Autonomous Spacefarers, slowly mustered a large force in order to invade Earth and disband the Confederacy Senate. This culminated in the Sol Campaign where Sol aw the largest conflict it had ever encountered within its gravitational influence, and the Earth Campaign which triggered a massive Kessler Syndrome, forcing a mass-scale evacuation of the planet. Due Commonwealth's decisive defeats in the Battle of Ius River, Battle of Venus-Mercury, and Battle of the Persian Gulf, it was repelled from the system and further away from the core territories. However, a three-year war in its capital had exhausted the Confederacy enough to virtually withdraw any resistance to the secessionists. It later extended the ceasefires which were accepted (opposing parties knew that invading a Confederacy planet will still result in a human alliance to force them back). In this time period, seceded states began to draw territories and borders from each other. They were finally made officially recognized entities past the Treaty of Tuuze, Article 8:
"Systems, Planets, Territories, or combinations of multiple of such, which had declared separation from the Confederacy prior to this treaty, shall be recognized as independent entities with lawful sovereignty over the space, air, waters, and lands it administers and controls."
It was not long before these states began to declare war and annex one another. The following years were also known as the Reunification Centuries, where some of these states grew from several million square kilometers (or less) of territory into powers in the galaxy. The Confederacy did not do much military action against these states (with exceptions), and focused in colonizing uninhabited systems within its borders or unclaimed space instead.