The Unified Earth Government traced its origins back to the Interplanetary War of the mid- to late-22nd Century in the Sol system. With numerous Earth governments fighting independent battles against Frieden and Koslovics forces, the UN became involved, eventually using the war to absorb most, if not all Earth governments into itself, forming a global government to combat the terrorists and stabilize the planet and its other world colonies. The UN won the war and the Unified Earth Government was formed, with its' primary defense, scientific and exploratory force being the newly formed UNSC. Although the UEG seemed to have complete control over Earth and its colonies, the UN still existed as an organization and a political entity in the 26th century.
“Humanity can now breathe again. The Covenant has finally been driven back. The cost in lives- our troops and our citizens- has been enormous. But freedom never comes cheaply, and now, we rebuild. I promise this to every man, woman, and child on Earth and in its colonies. While we will continue to strive for a peaceful coexistence with other species, humanity will never again allow itself to be the victim of aggression. This is the moment we start to reclaim our rightful place in the universe.”
— Inaugural speech of Dr. Ruth Charet, new President of the Unified Earth Government; January 2553.
Following the end of the war and the inauguration of Dr. Ruth Charet the UNSC handed back civilian powers on January 8, 2553. As well, the UEG began Project Rebirth to help rebuild parts of Earth after the destructive battle for the planet.
The UEG's influence over these worlds were greatly reduced by the Covenant during the Human-Covenant War, with only Earth and a few scattered colonies left mostly intact by the end of the conflict in 2552. Since the end of the conflict, the UNSC is working to restore its galactic territories and has even recolonized some of its former worlds.
Both the Unified Earth Government and the United Nations Space Command are references to the human government (United Earth Government) and military (United Earth Space Council), respectively, from Bungie's Marathon series.