The mainland of the Federation.
The United Federation was one of the great powers of Mobius prior to the Great War. A constitutional, democratic republic, its politics were often followed by many as it grew in prominence during the Industrial Revolution.
As the cradle of the Revolution, it far outpaced others in terms of its technological developments, decades ahead in practically every development. The noble house of Kintobor, however, would soon give rise to the famous Julian Kintobor, who turned that advantage into centuries. Julian would develop such things as the internet and laser technology with his great genius.
At the same time, the Federation was in a state of upheaval, nobility fighting for dominance with big business and the masses. As time went on, the nobility would be gradually erased from the picture; this socioeconomic shift would embitter many of the former nobles, who would clamor for a chance to regain their former power...
Eventually, he who made the Federation so strong became its worst enemy - New Years' Eve of 1199 , Julian staged a coup and overthrew the government of the Federation. Within a few months, his expansionist agenda had triggered a global war, in which the Federation and its scant allies emerged victorious, the enemy alliance incapable of matching the UF's technology.
The Federation annexed many of the lands it seized. Shortly afterward, the Federation was renamed the Robotnik Empire, and its democracy gave way to a dictatorship.
Trivia
As a stark contrast to the Acorn Kingdom, which draws influences from European monarchy and medieval themes, the Federation derives its influences from the United States and industrial themes.