The Great Cybertronian War (commonly referred to as The Great War) is an event that defines the multiverse of the Transformers. It is an ages-long conflict between the two main factions of Transformer society: the Autobots and the Decepticons. The war has always taken place in some way or another in every Transformers continuity. In the Generation One continuity, the war began when Megatron, leader of the Decepticons, rallied numerous disgruntled labourers, gladiators and criminals in the city-state of Kaon to form a rebellion against Cybertron's corrupt government. Cybertronian society quickly became divided, with everyone required to take sides. Those who tried to remain neutral were either regarded as enemies to both sides and killed, or were forced to leave the planet.
As the war escalated, the Decepticons seemed to ignore the original purpose behind their rebellion, focusing on the conquest of not only Cybertron but other planets as well. As new battlefronts were established, Cybertron's resources were drained until there was virtually nothing left, and the planet had been reduced to a dead wasteland. With their home planet all but destroyed, the Transformers scattered throughout the galaxy in hopes of finding the resources necessary to rebuild their home. This would go on for literally millions of years and Cybertronians would come to be feared and hated by many races across the universe.
Most notably, the Great War seemed to enter its twilight phase with the discovery of Earth. The Decepticons were determined to harvest Earth's energy resources and destroy its native human population. The Autobots, while still mistrusted by many of Earth's governments, have sworn to protect Earth's people and have made several alliances among the humans, being granted new resources and areas to operate from. Officially, the war has now ended, but Earth remains the last front where the Autobots and Decepticons continue to fight.