Unicron Trilogy
The Decepticons appeared in Transformers: Armada, Transformers: Energon, and Transformers: Cybertron as mostly jets and military vehicles.
In the Unicron Trilogy continuity, the early history of the war is not expounded upon. The Decepticons, however, remain the main antagonists, with whom the Autobots have been warring for quite some time. The enslavement of the small, power-enhancing Mini-Cons has long been a major goal of the Decepticons, and the Mini-Cons are seen as key to winning the war.
Near the end of Armada, the Autobots and Decepticons join forces to defeat the planet-devouring Unicron, and the beginning of Energon sees them as uneasy allies. Many return to Megatron's side when he returns.
Cybertron sees a much smaller Decepticon force than the previous two series, and what has become of most of the survivors of the earlier series remains unknown. As Megatron's true plans are discovered, most of the Decepticons, unwilling to be sacrificed as pawns as their leader seeks to remake the universe, again battle a common foe alongside the Autobots. The end of the series sees three of the comic-relief characters, with a reluctant Thundercracker, desiring to begin a new Decepticon force, and take off in a makeshift rocket. It makes it as far as Mars before it crashes, as the ending credits montage of the final episode shows. The foursome survives, however. The fates of the other surviving Decepticons remains unknown, though evidence of Starscream's survival is discovered.
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