Cybertronian legend tells of an ancient, eons-raging battle between Primus and Unicron. After Primus had cast Unicron into space by creating the thirteen original Transformers, Unicron slumbered for many ages. In his slumber, debris gravitationally attached to Unicron formed the planet Earth, of which Unicron is its core.
Late in the Cybertonian civil war, both the Autobots and Decepticons hid their energon stores off world, a practice that seeded planets like Earth with its deposits of the precious resource. In doing so, however, the war spread to these alien worlds, with the largest Cybertronian battle in the Milky Way galaxy being fought on Earth. The aftermath of this bloody conflict was left undisturbed for thousands of years, and was not common knowledge amongst most Autobots.
As Cybertron began to shut down to purge the Dark Energon corruption, the scientist Shockwave began using space bridge technology to try and find a large cache of energon that could be used to prematurely reboot Cybertron. His quest eventually brought him to Earth, where the dominant life-forms inspired him to experiment on four captive Autobots, producing the Dinobots.
Following Optimus Prime and Megatron's initial skirmishes on the planet, the former ordered all Autobots to Earth, while Starscream was sent to salvage a space bridge from wherever he could find it. Transformers: Prime The Autobots, meanwhile, set up Autobot Outpost Omega One in an American missile base and opened up relations with the United States government: Special Agent Fowler of the Pentagon was assigned as their liaison.
Starscream built his space bridge to bring Decepticons to Earth, but Arcee and Cliffjumper sabotaged it; only Starscream came through. On Earth, Starscream rendezvoused with Breakdown to assault the two saboteurs, but Optimus and Bumblebee rescued them. Returning to base via a GroundBridge, Optimus informed Cliffjumper more Autobots were gathering, and the battle had only begun. Transformers: Prime The Autobots would covertly watch for Decepticons, while the 'cons covertly mined energon.
The Covenant of Primus tells of an eternal conflict caused when certain planets come into alignment. The Autobots figured out that Earth was the planet that was prophesied as chaotically formed, as did Megatron. During Optimus Prime's one-on-one battle with Megatron, the volcano setting erupted with Dark Energon, right when the planetary alignment as predicted came to pass. One Shall Fall Earthquakes and tornadoes hit across the planet right after the volcanic event. It was soon determined that this weather phenomena was caused by the awakening of Unicron. Optimus Prime theorized that after Unicron was defeated by the thirteen original Primes, his dormant form collected space matter, which eventually formed into the planet Earth. Optimus Prime also theorized that Unicron became formless during that time. At least, he hoped Unicron became formless. Otherwise, if he woke up and stretched, KAPOW! Earth would poof to dust. The strange weather events ended when Optimus Prime used the Matrix of Leadership to force Unicron into stasis.
After activating the Omega Lock, Megatron directed a shaft of energy through a space bridge, striking Earth near Jasper and creating a huge Cybertronian fortress. Agent Fowler had the town evacuated as a result. Optimus used the Star Saber to destroy the Omega Lock, saving Earth in the process. However, when the Decepticons returned to Earth, Megatron dubbed this fortress New Kaon before proceeding to destroy the nearby Autobot base.
When human military forces moved in, Megatron used the fortress's fusion cannons to devastate both them and most of Jasper. He had also by that time decided to call the fortress Darkmount. The Decepticons moved in shortly afterward. Darkmount, NV Smokescreen infiltrated the base using the phase shifter in an attempt to find the Forge of Solus Prime, which turned out to be stored aboard the Nemesis.
Once he arrived on Earth and learned of the situation, Ultra Magnus planned a mission to neutralize Darkmount in tandem with Earth's military. Despite drawing off a large portion of the Decepticon forces, the remnants of Team Prime were eventually overwhelmed. Fortunately, the timely arrival of Optimus Prime, recently upgraded, allowed them to turn the tide and disable the fusion cannons, allowing Fowler and his fighter squadron to bring the citadel down.
Many years later, the Autobot prison ship Alchemor crashed on Earth. The ghost of Optimus Prime told Bumblebee to assemble a new team to hunt down the escaped Decepticon prisoners.
Main Article: Earth/Gallery