Atrions claimed the Zeons shared their physical appearance (humanoid, indeed), despite having a different attire.
Prior to the Atrion-Zeon War, the natives of Zeos had a relatively good relationship with their neighbours on the planet Atrios. This eventually deteriorated, leading to war between the two planets. It later turned out that the planet was totally depopulated, for unspecified reasons. (TV: The Armageddon Factor)