"Your leaders will now understand. Decepticons will never leave your planet alone. And we needed them to believe we had gone. For today, in the name of freedom, we take the battle to them!" -Optimus Prime
The Battle of Chicago was the third major battle between the Autobots and Decepticons on Earth during the Second Cybertronian Civil War. The battle occurred in Chicago shortly after it was invaded and fortified by the Decepticons.
The battle could be traced back to the final conflicts of the First Cybertronian Civil War, where the Autobot starship, the Ark, carrying with it the Cybertronian Pillars attempted to escape the war so that the Pillars could be used for refugees from the planet. In truth, that was just a falsehood, the reality was that former Autobot Sentinel Prime and Megatron had bartered a deal that would bring the Pillars to a planet where there would be manual laborers to help rebuild the ruins of Cybertron, (i.e. humans). However, the ship was damaged during its escape where it crashed on the moon where the Cybertron ship would remain there for ages until the first humans landed on the moon and discovered the ship.
In 2011 the Autobot-Human Alliance organization NEST would arrive at Chernobyl while hunting the Decepticon Shockwave. After skirmishing with the deadly Decepticon NEST uncovered, much to Optimus Prime's disbelief, a power core to the Ark. This would earn Optimus' anger and lead the Autobots to take their ship to the moon where they would recover the body of Sentinel and bring him to earth along with five Pillars that were seen to the scouting party on the ship along with the control Pillar, then bring them back to Earth, where Optimus would use the Matrix of Leadership to revive the dead Cybertronian.