The Great War is the civil war on Cybertron between Autobots and Decepticons. It lasted millions of years and eventually made the home planet uninhabitable.
Different versions of the start
The 1984 cartoon keeps it short: the Decepticons want to conquer Cybertron and the Autobots defend it. Later versions add depth: Megatron leads an uprising against an unjust system, while Optimus tries to change the same system peacefully. Their former friendship, shown in Transformers One, makes the split even more tragic.
Why the war dragged on
Both sides were evenly matched and convinced they were right. Energon ran out, cities crumbled, yet neither faction could win for good. Eventually the Autobots left for space in search of energy and a new home — and the Decepticons followed.
Consequences
That's how the war came to Earth. In the classic story the Autobot ship, the Ark, crashes on our planet, where the heroes lay unconscious for millions of years until a volcano woke them in 1984.
