Prior to the Atrion-Zeon War, the Zeons had a relatively good relationship with their neighbours on Atrios. This eventually deteriorated, leading to war between the two planets. Unknown to the Atrians, Zeon was totally depopulated, meaning that either the Atrians had unknowingly succeeded in wiping out the Zeons at some point during the war, that the Shadow had deliberately wiped out the population, or that the Zeons had abandoned their planet.
Subsequently, Zeos was governed by the supercomputer, Mentalis, created by Drax, a Time Lord. Mentalis was programmed to self-destruct if threatened; the explosion would destroy Zeos and Atrios. The Fourth Doctor and Drax managed to disable Mentalis just before its countdown sequence completed.
The third planet blocked a view of Atrios from Zeos and vice versa.
The Doctor diverted an attack by the Marshal of Atrios away from the planet into the Planet of Evil. With the war over, the Doctor suggested that the Atrians would claim Zeos as their own and begin rebuilding the planet. (TV: The Armageddon Factor)