The Decepticons descended from the Quintessons' Military Hardware line of robots. Much of the early history of the Decepticons remains unknown, but they doubtless were instrumental in the overthrow of the Quintessons' rule and in the foundation of the golden age that followed the revolution.
However, the Decepticons (as they soon renamed themselves) came to believe that it was their destiny to expand Cybertron's dominion to other worlds, and to conquer the lesser races. The Autobots opposed this philosophy of expansion, and the Second Cybertronian War followed. The Decepticons had the initial advantage, but it was soon countered by the Autobots' development of transformation technology. However, the Decepticons replicated and improved this technology.
An uneasy peace followed the Second Cybertronian War, but the Decepticons secretly created their ultimate weapon- Megatron. Megatron led the Decepticons to destroy the current Autobot leader, and started the Third Cybertronian War. Megatron was the greatest leader the faction ever knew, until he led the Nemesis to attack the Ark around 4 million years ago. Both ships crashlanded on prehistoric Earth, and Shockwave was forced to assume leadership of the Decepticons in his absence.
Megatron and his warriors awoke anew in 1984, and quickly took advantage of the energy-rich planet they landed on, making plans to strip Earth of its resources so that Cybertron could be revitalized. However, Optimus Prime and his Autobots continued to oppose them. Despite this, Megatron led the Decepticons to a conquest of Cybertron by 2005, forcing the Autobots to relocate to the planet's moons.
Unfortunately for the Decepticons, Megatron chose in 2005 to lead a sneak attack on Autobot City on Earth, which led to a climactic battle that slew many warriors and left Megatron near death. Starscream, who had always coveted Megatron's position of leadership, seized the opportunity and left Megatron and the other dying warriors for dead in space, where they were encountered by Unicron. In exchange for destroying the Autobot Matrix of Leadership, Unicron recreated Megatron and his warriors into Galvatron, Cyclonus, Scourge, and the Sweeps. Galvatron destroyed Starscream and retook leadership, leading the Decepticons to hunt Ultra Magnus and the Matrix within him. However, Galvatron attempted to break free from Unicron's control using the artifact when he acquired it, leading the planet-eater to attack Cybertron. The Decepticons defended their homeworld, but it was the power of the Matrix that destroyed Unicron and threw Galvatron across the galaxy, landing in a lava pit on the planet Thrull.
The battered Decepticons were forced off-planet by Rodimus Prime and the Autobots, and retreated to the burnt-out world of Charr. Cyclonus barely managed to hold the energon-deprived, demoralized Decepticons together for another year, before he finally found Galvatron. Unfortunately, his leader was now insane from a year of lava exposure, and Cyclonus continued to be almost all that kept the Decepticons together. Under Galvatron's leadership, the Decepticons became a frequent pawn of the Quintessons.
Events then continued in exclusive Japanese series: Headmasters, Masterforce, Victory, Zone, Battlestars, Mission-Combination, Beast wars 2 and Beast Wars Neo.
One of two factions of the Transformer race, devoted to expansion and the conquest of all lesser races. The Decepticons had their roots in an individual known as the Liege Maximo. This Transformer, via the process of bio-morphic reproduction, was reponsible for creating the first Decepticons, possibly including Megatron. He left Cybertron sometime in its primordial past.
Megatron became Decepticon leader, and eventually started the Great War, to eradicate the Autobots and the stagnation and decay he believed them to represent. The battle raged, with the Decepticons slowly gaining ground, until Optimus Prime appeared and the tide began to turn more to the Autobots' favor. A thousand years of fighting continued with no clear victor, until the strongest warriors on both sides were lost on the planet Earth 4 million years ago.
Without Megatron, the Decepticons broke down into several sectors dominated by powerful warlords, such as Lord Straxus' Polyhex and the territory of Lord Jhiaxus. Jhiaxus determined that the Decepticons' true destiny lied in expansion, rather than defeating the Autobots over a sentimental attachment to a dying homeworld. To that end, he and his followers built the spacecraft Twilight and left Cybertron. After rediscovering the process of bio-morphic reproduction, Jhiaxus' group became the Second Generation Decepticons. Straxus became the dominant Decepticon force on Cybertron.
In 1984, Megatron and his forces reawoke, and began pillaging Earth for its resources. However, tainted fuel created by "Sparkplug" Witwicky took out Megatron's warriors just as they were about to finally destroy the Autobots led by Prime. Shockwave took the opportunity to assume leadership, resulting in nearly two years of leadership battles between himself and Megatron, and the re-establishment of contact with Cybertron. Meanwhile, Straxus was destroyed in battle with the Autobot Blaster, leading to Ratbat's tenure as leader on Cybertron.
Megatron was able to bring about Prime's destruction, but, unable to deal with the circumstances involved, he apparently destroyed himself. Shockwave became leader for a few more months, before Ratbat stepped in and took over. Meanwhile, Decepticons led by Scorponok pursued Fortress Maximus's forces to Nebulos and became binary-bonded to some of the inhabitants. This group then travelled to Earth, again following Maximus, and both groups of Decepticons remained ignorant of one another for some time.
In a battle with Fortress Maximus, Shockwave fell into Earth's atmosphere, and vanished. During the Underbase crisis, Scorponok became leader of all the Decepticons on Earth by destroying Ratbat. However, Scorponok led the Decepticons into a period of lengthy inactivity, resulting in the perfect opportunity for Shockwave (who had survived) to assemble a group of renegade Decepticons in an attempt at takeover.
In the meantime, Thunderwing had gained power on Cybertron by this time, and attempted to find the Creation Matrix, a quest that resulted in his possession by the corrupted Matrix and death. Shockwave made his attempt, just after Optimus Prime surrendered the Autobots to the Decepticons in an attempt at union against Unicron. Shockwave's faction was trounced, but Primus pulled all Transformers on Earth to Cybertron, where they joined forces against the Chaos-Bringer. In the process, Scorponok was destroyed, and Bludgeon became Decepticon leader.
Bludgeon led his warriors to end the alliance with the Autobots and try to conquer the planet Klo, which was narrowly stopped by the Autobots and (mainly) the Last Autobot and a resurrected Optimus Prime. Bludgeon and his forces built the mighty Warworld, and became the terrors of the galaxy. He eventually concocted a plan to create new soldiers by luring Prime to Earth, and stealing his Creation Matrix. However, a Cobra-reconstructed Megatron ended his dream by retaking leadership. Megatron proceeded with Bludgeon's plans, until he came into conflict with the Second Generation Decepticons, which led him to ally with the Autobots against them. The alliance remained after Jhiaxus was defeated.
The faction was brought together by Megatron, aided by his fame as a gladiator. In the 20th century, a clone of Megatron tried to reunite the disparate Decepticon factions on Cybertron, but committed suicide when he realized he was a fake, passing his work down to the real Megatron.
The last Decepticons on Cybertron were ultimately destroyed in 2510, but the final survivor, a spy among the Autobots, gained revenge by pitting the remaining Autobots against one another.
Alternate Marvel UK/US Timelines
The original timeline led to the Decepticon conquest of Cybertron, similar to what is seen in the U.S. animated canon, as well the events surrounding Unicron's attack (although the events actually took place one year later than in U.S. animated canon). Galvatron, Cyclonus and Scourge timejump and thus create the main timeline. Galvatron made several attempts at altering history in his favor, first during his servitude to Unicron, second after he had been tossed by Rodimus Prime (the damage from his landing in the present made him insane), and a third time (having remained in the present since his second trip) that led to the Time Wars. Galvatron was destroyed at the end of this. After Galvatron's disappearance in 2006, Shockwave took over the Decepticons again, until he was assassinated by Death's Head (unwillingly, as Death's Head was under the control of Unicron). Soundwave became tentative commander of the faction after that.
The original timeline was eventually erased by a space-time rift and replaced by alternate future, shown in 'Rhythms of Darkness'. Like before the Matrix was passed to Hot Rod, making him Rodimus Prime - however, Galvatron killed him, Unicron attacked again, and the Autobots were reduced to a tiny force of less than a dozen members by 2009. Most of these died attacking the Decepticons' base in an attempt to prevent a nuclear strike from the remaining free world powers on Earth. However, that universe's Galvatron was taken cross-time to help the 1990 Unicron, and the Autobots gained a fighting chance. The fate of that universe's Decepticons is unknown.
The Decepticons were defeated by combined human-Autobot forces in what they termed Operation Liberation. In 1999, humans and Autobots built the Ark II, and the Decepticons were loaded on as prisoners to be taken back to Cybertron in the ship. However, it was sabotaged and exploded in mid-air, destroying the Autobots, the Decepticons, and the human envoys aboard. The Decepticons were reconstructed by human mercenaries, working for General Hallo, to use as weapons of war, but Megatron managed to regain self-control, followed by his other reconstructed Decepticons. Having had quite enough from the humans, Megatron engineered a cyber-virus, spreading from the mercenaries' base outward, that would turn everything in its path to machinery. (It was ultimately halted by Wheeljack.) Then, he led his troops to attack San Francisco, only to be intercepted by Optimus Prime and his Autobots. Grimlock, swayed by Megatron's rhetoric about the folly of helping humans, briefly joined the Decepticons (but later was back on the Autobot side). Narrowly escaping nuclear destruction thanks to the sacrifice of Superion, both factions escaped to fight later.
The Decepticons soon learned that more had changed on Cybertron; the Great Shutdown led Autobots and Decepticons to unite under the leadership of Shockwave (Transformers), but the alliance crumbled when the Earth Transformers were captured and Shockwave's true plans for power were revealed.
Similar events experienced by the Decepticons in the U.S. animated continuity surrounding 2005 occurred, followed by Galvatron's time as Decepticon leader and they were later joined by the Hive of Nebulos.
The Decepticons fared worse and worse, until they were defeated by the Autobots and forced into suppression by the Pax Cybertronia. After this point, the Decepticons became the Predacons, and most major Decepticons were tried for their war crimes. Some, like Ravage, were granted amnesty, but most were probably imprisoned or worse.
The conclusion to the war is shown in the 'Beast Wars' and 'Beast Machines' animated series.