The Borg-Cyberman Alliance was an alliance between the Borg Collective of the Federation universe and the Cybermen who crossed over from N-Space, providing their Cyber-Armada to form the Borg-Cyberman Fleet. (COMIC: Assimilation²)
The Borg-Cyberman Alliance made its first appearance to the United Federation of Planets in 2368, when their fleet invaded the Federation planet of Delta IV in the Delta system. Quickly overruning the planet and the Starfleet forces there, they forced the surviving ships into a hasty evacuation.
Soon after, the Starfleet vessel USS Enterprise (NCC-1701-D), having received a distress signal from Delta IV, arrived in the system where they discovered the Borg-Cyberman Fleet. Immediately subjected to an attack, the Enterprise was forced to flee, hiding in the Paertes Nebula. As anticipated, the fleet was hesistant to follow them into the nebula.
The fleet then set off, their flight path seemingly directed towards Earth's solar system, before stopping to assimilate the planet Cogen V. However, relations between the Borg and the Cybermen had already begun to break down. After detecting the Eleventh Doctor and the TARDIS aboard the Enterprise, the Cybermen were concerned with the threat he posed and, after acquiring the Borg's executive library routines in the Collective's core memory, turned on them whilst in the midst of assimilating Cogen V. Overwhelming the Borg vessels, the Cyber-Armada broke off and proceeded on a course to the Borg homeworld in the Delta Quadrant. (COMIC: Assimilation²)
With the grudging assistance of the Enterprise crew and the Doctor, the Borg were able to overcome the Cybermen. Gaining access to the Cyber-web, the Borg set the entire Cyber-Armada to self-destruct, ridding their universe of the Cybermen incursion. Through this encounter, however, the Borg had become aware of the Time Lord, and sought to master time travel in order to assimilate him. (COMIC: Assimilation²)