United Nations Space Command

The UNSC or United Nations Space Command, is the prime Human government in the Halo universe.

Formation
The United Nations Space Command was formed as the result of a series of brutal wars across the Sol system between 2160 and 2164, most promiently among them the Jovian Moons Campaign, the Rain Forest Wars, and a series of battles on Mars. These conflicts were centered around UN clashes with dissident political movements, the most important of which were the "Koslovics" and the "Friedens." The Koslovics—supporters of neo-Communist hardliner Vladimir Koslov—sought a return to the glory days of Communism and the elimination of corporate and capitalist influence, particularly in orbital facilities and offworld colonies. The Frieden movement was a resurgence of fascism, springing from anti-Koslovic sentiment that had taken root in the Jovian colonies. "Frieden" literally means "peace" - in this case, they believed that peace could be achieved only once the "oppressors on Terra Firma" were eliminated.

Lasting over four years, the massive wars raged across Earth and in numerous off-planet colonies. The brutal three-way war between UN, Koslovic, and Frieden forces sparked a massive buildup of arms and troops within the United Nations. Utilizing aggressive space-based Marine drops, the UN defeated Koslovic forces on Mars, which would pave the way for the future incorporation of Marine units as primary ground combat elements. It was at this point that the United Nations Space Command was first created to oversee UN deployments across the solar system. The UNSC proceeded to use its overwhelming military might to crush all remaining Koslovic and Frieden holdouts across the system, ending with a single Unified Earth Government under the control of the UNSC.

Interstellar Colonization
Massive overpopulation and a huge military with no enemy to fight presented immediate problems for the postwar United Nations Space Command. Utilizing the newly developed Shaw-Fujikawa Slipspace Drive, in 2310 the UNSC deployed a vast line of colony ships to colonize the nearest planets and systems, giving birth to the "Inner Colonies." Vast numbers of citizens and soldiers travel to these new colonies, stabilizing the overpopulation problem on Earth and giving the military new tasks to handle. As the first colonists are stringently selected, the Inner Colonies eventually form an "elite" society of the best physical and mental human specimens.

By 2490 the UNSC had colonized over eight hundred worlds in the Orion arm of the galaxy.(ranging from fairly well-tamed planetary strongholds to tiny hinterland settlements). Outward expansion continued, and the Inner Colonies become a political and economic stronghold, though they relied heavily on raw materials supplied by the "Outer Colonies."

During this period, the planet Reach (orbiting Epsilon Eridani, right on Earth's metaphorical doorstep) became the UNSC's primary Naval yard and training academy. Reach was a major producer of warships and colony vessels, as well as a training ground for covert operatives and Special Forces.

However, this expansion did not come without cost, as ideological and political causes resulted in many of the Outer Colonies engaging in open rebellion. A series of brushfire wars across UNSC space sprung up, with hundreds more threatening to erupt as well. by 2517, UNSC estimates put massive, overwhelming and uncontrollable rebellion at less than twenty years away. This resulted in the start of the Spartan-II program, to quickly and quietly deal with insurrections without incurring massive civilian casualties.

Covenant War
On October 7, 2525, A UNSC battle group consisting of one Destroyer (Heracles, Captain Veredi) and two Frigates (Arabia and Vostok) enters the Harvest system and discovers that everything on the planet has been completely incinerated and the surface melted down to glass. In orbit around the remains of the colony world is a single alien ship, which proceeded to attack the battle group, destroying both Arabia and Vostok and badly damaging the Heracles. Intercepted communications identify the enemy as The Covenant. The message "Your destruction is the will of the Gods... and we are their instrument" is broadcast in UNSC language.

Vice Admiral Preston Cole mobilizes the largest fleet in human history to retake Harvest. Cole's war fleet engages the alien warship responsible for the decimation of the colony, scoring a victory (though the battle cost Cole two-thirds of his battle group). Only a last minute tactical inspiration turned the tide of battle. After returning to Earth, Cole-promoted to Admiral-learns that a number of outlying colonies have been destroyed, leaving behind no survivors. Cole begins to move his fleet around, trying to intercept the invaders. Ground and ship-to-ship battles begin in earnest, raging throughout the Outer Colonies.

By 2535, Cole's forces are hammered, despite his excellent leadership and tactical brilliance. It is simply a matter of being outgunned; Covenant kill ratios tend to top four to one in ship-to-ship combat. By 2535, virtually all of the Outer Colony worlds have been destroyed by the Covenant. The Cole Protocol is established by military order: all human vessels must ensure that Covenant forces do not find Earth.

In 2536, Covenant forces begin to swarm into the Inner Colonies. For several years the war falls into a pattern: humans win isolated battles, typically during ground operations, but at a horrible cost. In space combat, the humans are losing at a precipitous rate, and one by one, the colonies fall. 

By 2552, human forces are reduced to handful of planets near Earth, with Reach as their last stonghold. Covenant forces follow a UNSC destroyer with a sophisticated tracking device to Reach, and deploy a three-hundred ship fleet against the planet. Despite the heroic efforts of the human defenders and their massive orbital MAC stations, they are unable to prevent Covenant forces from landing on the planet, disabling planetary power generators, and leaving Reach helpless. The majority of the planet is glassed, and the fleet defending it is nearly totally wiped out.

Shortly after this battle, the Master Chief, on board the Pillar of Autumn, participated in the events of Halo: Combat Evolved. In the aftermath, he and Cortana, along with a small squad of human survivors captured a Covenant flagship and pillaged the technologies within. In the process, they discover a massive fleet of Covenant ships heading toward the Sol system, and proceed to sabotage and destroy the fleet in a daring and costly assault.

On October 12, 2552, UNSC defense forces over Earth engaged the Prophet of Regret's fleet and destroyed most of it, in the process discovering that the Covenant had not actually located Earth, and instead had apparently stumbled across it unexpectedly. During the ground battle in New Mombasa, Regret makes an in-atmosphere slipspace jump, which is followed by the Master Chief and the frigate UNSC In Amber Clad to a second Halo. After the events on Delta Halo, the Master Chief returns to Earth on board a Forerunner ship to find it under attack by massive, overwhelming Covenant forces.

The announcement trailer to Halo 3 showed Earth thoroughly conquered at least in Africa and UNSC forces apparently overwhelmed by the Covenant onslaught there. It is unknown what or how many UNSC forces remain on Earth or how many colonies and ships remain intact at this point.