Cyber-Wars

The Cyber-Wars was a galactic conflict in which an alliance made a concerted effort to wipe out the Cybermen in 2526. A force of time-travelling Cybermen from the end of the conflict attempted to prevent this. They tried to destroy the Earth, first with a Cyber-bomb and then by crashing a human freighter into the Earth. Both of these attempts were stopped by the Fifth Doctor. (TV: Earthshock)

It was during this conflict that the glittergun was developed. Gold was a major weakness of the Cybermen. The gun could fire gold dust at them.

The wars resulted in near-complete destruction of the Cyber race. Remnant groups of Cybermen fled across the galaxy. (TV: Revenge of the Cybermen)