Unified Earth Government

The United Earth Government, or UEG is the central civilian government that administers Earth and its colonies throughout a portion of the Orion Arm of the Milky Way galaxy. It houses the Colonial Administration Authority, and through it the United Nations Space Command.

History
the United Earth Government traces 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 Koslovic forces, the United Nations became involved, eventually using the war to absorb most, if not all, Earth governments into itself to form a global government to combat the terrorists and stabilize the planet and its offworld colonies. The UN won the war, reforming itself into the UEG, with its primary defense force the newly formed UNSC.

Colonial Administration Authority
The Colonial Administration Authority is the branch that administers the various colonies that belong to the UEG, incorporating the UNSCDF. It keeps extensive files on every colony world and city under the UEG, available to UNSC members for planning missions.

United Nations Space Command
The UNSC is the UEG's primary defense force, intended to protect colonists and colony worlds from attack by privateers, Insurrectionists, and other threats, such as the Covenant. It includes various special forces, and militia organizations, but is mainly comprised of the Marine Corps, Navy and Army. It is also the UEG's main exploratory and scientific arm.

Trivia

 * It should be noted that both the UEG and UNSC are references to the human government and military in the early Bungie game, Marathon. See List of Marathon references in Halo.
 * The UEG is only ever mentioned in the instruction manual of Halo: Combat Evolved.