The world of the Universal Century has continuously been built on for 40 years and at the center of it all is the Earth Federation— A monolithic, supreme keeper of order. Centuries from now, in the future that is the Universal Century, the Earth is so overburdened with overpopulation and extreme environmental degradation that the only course of action is to bring the vast majority of the human population into space to live out in space habitats called Colonies. To achieve this goal and to save Earth, the nations of the world banded together to form the Earth Federation, and at its signing marked the start of a new calendar, marking the start of the year UC 0001. In UC 0079, the single most devastating war in history would occur, the One Year War. The Principality of Zeon hailing from the Side 3 started a war of independence and with the help of their new revolutionary new weapons called Mobile Suits, they would be able to fight and win against what was thought to be an invincible Earth Federation for the first 6 months of the war, which culminated in the loss of half of the population in the Earth Sphere with more than billions dead. The Earth Federation eventually made their own mobile suits and pushed back the Principality of Zeon and just before the end of the year UC 0079, peace was finally achieved. In the following decades, conflict will riddle the Earth Federation time and again, with even more lives lost. It is said that in the far, far future, the once-monolithic Earth Federation will be no more, just to be forgotten and become nothing but a footnote in history.
Brief History of the Earth Federation written by me, Joseph Jobo Licayan. There is so much more worldbuilding done on this world with more than 10 TV shows, films, games, etc. The most important bits can be found here in the Gundam Wiki: