The Cyberman-Borg War was a war between the Mondasian Cybermen and the Borg that lasted for over 100,000 years. The conflict started when the two races, bent on conquest, met on a desolate planet that would be forever known as Hell's Doorstep. The two races initally talked about makig peace but, for reasons unknown, they started attacking each other.
The two massive fleets went to war, and the Borg Cubes and Cyberships engaged each other all over the Galaxy. Reports of some conflicts even reaching the inner colonies of the Galaxy are unconfirmed but very possible.
The Cybermen initially came off worst. The Borg, being superior in their ability to assimilate organisms, heavily outnumbered the Cybermen. Not only that, but some Cybermen even fell victim to assimilation themselves. The Cybermen had no way of upgrading the Borg.
However, soon the tide began to turn. With their backs to the wall and with nothing to lose, the Mondasian Cybermen called for help from the one civilisation in the Galaxy that would help them: the Cyberman Remnant. Being very similar in design, the two factions of Cybermen swapped data, technology and resources to make a massive combined fleet.
Faced with overwhelming numbers and suddenly outgunned, the Borg were pushed back. Their lack of ability to form alliances with anyone made the Borg weak, and not only that, but the Cyberman Remnant had provided the Mondasian Cybermen with an immunity to Borg Assimilation, small nanobots that circulate around the body and prevent assimilation. The Borg began retreating, but they were too late. The Combined Cyber Fleet attacked in full force, decimating the Borg Fleet. Scout ships sent by the League encountered only burnt-out husks of Borg Cubes, the ships that destroyed them having long gone.
If that weren't enough, in desperation, the Borg began attacking planets to find more people to assimilate and happened upon a Covenant Fleet. The Covenant ships glassed the Borg Cubes and managed to destroy an entire battlegroup with only three ships. The Cybermen later thanked the Covenant by giving them the Borg Immunity too. This soon spread around the Galaxy, as Jackal pirates sold the nanobots to every civilisation they could find.
The Borg were surrounded on all sides. Once they were feared, now they were fearful. Angry civilisations cried out for action. Even the Daleks and the Reapers joined in. The Cyber Fleet, now on the brink of annihilating the Borg completely, were reinforced by Dalek Ships, Reapers and League Assault forces. Not only that, but the Borg Queen, in an attempt to invade the Human homeworld Earth, had been killed by the crew of the USS Enterprise, spearheaded by Jean-Luc Picard.
Now one combined species, the Cybermen had shown their true power to the Galaxy and, in their hour of need, the Galaxy had supported them. In one final battle, during which the Cyber Fleet led one final assault on the Borg SuperCube, the Borg were utterly annihilated. Celebration erupted all over the Galaxy. The Federation attached massive banners to their ships declaring that Resistance ISN'T Futile! And the Cybermen were given a bigger place in League society. Not only that, but this war saw the melding of the two Cyberman races into one Superace.