Fort Acorn was created following Robotropolis' destruction by a Station Square nuclear strike, in order to protect the dome shield which contained the harmful radiation from the warheads. The fort contained the Mes Braves Battalion , which was commanded by the famous general Armand D'Coolette. However, Fort Acorn became a battleground following Sonic the Hedgehog's return from a year in outer space, as Dr. Eggman launched an armada of SWATbots to bring down the dome shield, in order to allow the radiation to spread. His SWATbot horde was ultimately destroyed however by the combined might of the Mes Braves Battalion, the Chaotix, G.U.N. agents and Amy Rose.
The Iron King later invaded Fort Acorn in an effort to destroy Robotropolis' energy converter. Despite the best efforts of Amadeus Prower and the Mes Braves Battalion, they were overpowered by the Iron King, who then proceeded to destroy Fort Acorn, and head onwards towards Robotropolis.