Unified Earth Government

The Unified 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, the United Nations Space Command, as well as the United Nations.

During the Human-Covenant War, the UNSC has assumed the governing function of the Unified Earth Government in the face of overwhelming Covenant threat. It is not known whether the UEG is still present or, if it is, what its remaining functions are.

History
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 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 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 United Nations still existed as an organization and a political entity in the 26th century. There are still numerous national governments under the UEG's power, although their overall authority is negligible as most governing functions are handled by the UEG.

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 innocent colonists and colony worlds from attack by privateers, Insurrectionists and other unknown threats at the time, such as the Covenant. It includes various special forces, and Militia organizations, but is mainly comprised of the Marine Corps, Navy, Air Force and Army. It is also the UEG's main exploratory and scientific arm.

United Nations
Even though it originally formed the UEG, the United Nations has been absorbed into a branch of the Earth government by the 26th century. Its exact function or role is unclear, but it is assumed that it still exists in the same manner as it was originally created.

Trivia

 * Though the UN and the UEG are functionally separate entities, the name "United Nations" was used to refer to humanity's central governing agency in Halo: Contact Harvest.
 * Both the Unified Earth Government and the United Nations Space Command are references to the human government (United Earth Government) and military (Unified Earth Space Council), respectively, from Bungie's Marathon series.