Former interstellar human government that lasted for 900 years before it fell to rebellion and war.
The URSS was established on Earth in 2100 as the human race began to scatter. At this point in time it was feared that a fragmented humanity would be susceptible to attack from outside and due to the discovery of the intelligent yet primitive (in terms of technology) Kya it was well known that life existed on other worlds. Of course, most humans were hopeful about alien contact as the first contact with the Kya had gone remarkably well. So the formation of the URSS was based off of exploration and expansion.
It was a republic with senators from each inhabited world. As the human race settled more worlds within its own system, the number of senators grew, so eventually the senate had to be moved into orbit.
Due to the government and military secretly fearing hostile advanced aliens, massive fleets were built by mining mineral rich asteroids and moons all across the ever expanding human dominated space. Of course the ships were mostly used as exploration vessels or for policing the near-by systems. The first hundred years of human expansion was, by most accounts, too fast. Humans settled worlds too quickly, occasionally leading to the entire colony being wiped out by some strain of alien microorganism. However the government put new laws in place to study the planets extensively before human settlements could be approved.
The first 200 years of the URSS experienced an explosion in human colonization, population and exploration. The URSS could barely keep up with the rapid expansion. They used large space-fairing companies to help build infrastructure and provide for populations on frontier worlds. These space-fairing mega-corporations had been established as early as the first private space station in 2027. By the year 2300 humanity's reach had spread as far as the Horse Nebula. It was in this time that major flaws in the URSS governing system began to become apparent. The URSS was unable to effectively govern any multi-planetary organization. In the Republic's constitution it was written that every inhabited world would have a senator in the Senate, so when corporations began to affect many dozens or even hundreds of worlds the corporations were able to indirectly or in a few cases directly control the votes that those worlds placed in the Senate. Muilti-planetary companies were able to manipulate the Senate into passing regulations that favored their businesses.
By the year 2450 the Senate had become a corporate battleground. Mega-companies fought against each-other in Senate to pass regulations that favored themselves. The largest and most powerful corporations used the URSS as a scapegoat and a legal guard-dog so therefore were virtually immune to any government regulations. By 2550 government regulations had ceased to favor the people and instead favored unsafe, cheap business practices. The quality of life within the Republic began to deteriorate due to immense corporate corruption. Tensions among the public rose and riots began to occur more and more frequently. In the year 2601 a terrorist group known as the Mintakan Army For Free Humanity emerged with a series of terrorist attacks on corporate or government facilities. Government and corporate security forces quickly quieted and captured a majority of the terrorists, but terrorist bombings became a frequent occurrence. In 2633 the URSS thwarted a terrorist plot to set off a nuclear warhead in the Senate. During the later part of the 27th century the riots became much more violent often times resulting in the execution on thousands of civilians. Then on March 4th, 2679 violent riots broke out simultaneously across hundreds of worlds, the most violent of which was on the planet of Mintaka. These became known as the Eastern Galactic Rebellions. The capital city and Terragen Cor. headquarters of Mintaka City was a warzone, thousands of civilians evacuating and Mintakan freedom forces fighting both URSS military and Terragen Security. thousands of people died in the first hour. By 4:27 pm local time a nuclear warhead was detonated just outside the Terragen headquarters. Half the city was leveled including the URSS command center. Thus marked the beginning of the Second Age.
On March 4th, 2679 hundreds of millions of people were killed. Major distant frontier worlds such as Martopia experienced violent and bloody riots while the inner core worlds stayed relatively safe, being subject to infrequent attacks. Terrorist groups emerged and proclaimed independence from the Republic. By the end of the day thirteen nuclear warheads had been detonated across nine different worlds. The URSS was stretched thin, the military had to abandon hundreds of worlds to consolidate on crucial core worlds. The scale of this terrorist, or now known to be rebel attack was immense and had likely involved decades of planning and preparing.
The Senate, in a state of emergency, handed all authority over the Republic to the military's Admiralty. The Republics Space Core (RSC) became ruthless now that they were able to ignore the URSS's humanitarian laws. The Freedom Fighters continued to fight and dig in, by the end of 2680 six more warheads were detonated and several of the previous mega-companies were disbanded due to the violence. With the disbanding of many hated companies the rebellious forces fought with increased fervor. The fighting continued even on Mars. During 2681 the RSC thwarted six terrorist plots to destroy the orbital Senate. In 2683, the RSC began to use ruthless tactics to bring a swift end to the war. The Republic's military began bombing and nuking planets that were associated with rebel forces or suspected of being associated with them. Millions of people were vaporized in nuclear explosions. By 2685 most rebel resistance had been crushed. Dozens of worlds across the Republic's empire were left as abandoned, radioactive ruins. The URSS was now run under the cold iron fist of the RSC's admiralty.
By 2700 most of the corporations of the previous era were a thing of the past. Many of them had been targeted and destroyed by rebellious or terrorist activities. The URSS empire was run by the ever expanding military. Basic supplies and support was few and far between, the quality of life within the empire was dismal. Over the next 200 years rebellion after rebellion flared up, the populace fighting for freedom against a government that was now more oppressive than ever before. Everytime a rebellion occurred the Republic's military responded with cold nuclear force. Humanity was a slave to a military oligarchy.
During this time the military sped up development of more powerful and more destructive weapons. Because humanitarian laws were no longer in effect the weapons were extremely dangerous and destructive.
In 2995 a split in the RSC occurred. One admiral, Admiral Luke McCollins defected from the Republic and brought with him a quarter of the Republic's military strength. His installations were immediately under heavy assault from the rest of the Republic, but his forces able to repel wave after wave of warships. Seeing the opportunity the Army for Free Humanity (AFH) terrorist group sought him out and joined forces with him. McCollins, along the AFH devised a plan to shatter the URSS. McCollins and the greatly bolstered AFH roused a new rebellion. War sparked on dozens of worlds, AFH individuals and military personnel loyal to McCollins fought against the ruthless RSC. In 2997 the AFH was once again on the verge of defeat from the RSC when their plan came to fruition. With McCollins help the AFH was able to smuggle a nuclear warhead into the senate. On June 6th, 2997 at approximately 3:15 am London time the Senate was detonated in a nuclear explosion. The admiralty and much of the RSC's commanding force were obliterated. The fighting continued, but now the URSS was decentralized and disorganized. Many worlds established their own independence and humanity became isolated and separated.
On February 3, 2999 one of the remaining weapons developers for the Republic, most likely out of patriotism for their government, launched an attack on the rebel planet of Stronghold, a major world that supported the rebel cause. The weapon caused massive tidal waves and earthquakes around the planet, causing the planet's volcanoes to erupt all at once, even the local supervolcano. Nearly all the inhabitants died and only a handful escaped. This incident became known as the Stronghold Incident, and it was a turning point in the war. When it was discovered that the weapon, if it hadn't been equiped with a limiter, would have destroyed Stronghold completely, all the remaining fighting stopped out of horror and fear on both sides. The patriotic military scientists themselves were filled with extreme guilt at their actions and after issuing a tearful apology vanished, most likely having committed suicide.
Due to the need for common regulation, trade policy, coordination as well as being influenced by the events of the Stronghold Incident almost every remaining colony sent representatives to a meeting on Terrasphere Alpha in orbit of Earth. There, on January 1st, 3000, they drafted a new, loose treaty and became the Confederacy of Borealis. The Confederacy was designed in a way that mean no multi-world organization could have more power than a single planetary government. Every organization and government had an equal say on what regulations and policies could be followed. The Federation was a loose collection of human settlements. If a colony wanted to defect they only needed a vote. Every organization or government was allowed to govern themselves however they saw fit so long as the Humanitarian Laws were followed.
As for the incredibly powerful weapons developed by the Republic (including the nicknamed Planet Buster) as well as the more traditional weapons used by the URSS during the war, it is rumored that they are stored in the Pluto Vault.