Decimators are large humanoid mecha designed for combat and are a major focus in the animated OVA, Super Dimension Century Orguss-02. In the OVA, these futuristic war machines are unearthed on a world and time different from the world they originate from. This is due to the detonation of a Space/Time Oscillation Bomb, which caused the fragmentation of space and time and led to multiple dimensions crossing over each other. This resulted in people and objects being separated from their home dimensions and displaced on other worlds.
In the 200 years since the space/time oscillation, Decimators have been discovered in an undeveloped world by different nations, who have attempted to restore the weapons they've uncovered and use them to wage war against their neighbours. The kingdoms of Revelia and Zaffrin are the two strongest nations of this world, both possessing large numbers of Decimators that they have unearthed. Both sides understand enough about Decimators to repair and maintain them, but neither Zaffrin or Revelia are capable of building their own robots. According to the sentient robot called The Colonel, the people of this world are supposed to invent Decimators far into the future.
In Orguss-02, Zaffrin and Revilia are warring with each other using their Decimator battalions, with Zaffrin apparently winning thanks to the discovery of a giant Decimator that is nigh-indestructible. Their war comes to an end when the Colonel builds a new Space/Time Oscillation Bomb and activates it, restoring all the dimensions to the way they should be, and thus all the Decimators disappear from the world.