From Halopedia, the Halo wiki
2160-2200: Early Conflicts
This period in human history was marked by a series of brutal conflicts between various governments and factions in our Solar System. Conflicts of particular historical importance included the Jovian Moons Campaign, The Rain Forest Wars, and a series of clashes on Mars.
As overpopulation and political unrest on Earth increased, a number of new political movements formed. The most noteworthy dissident movements of the period were the "Koslovics" and the "Frieden" movement. 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 (largely backed by Unified German Republic corporations, frequent targets of Koslovic "workers' crusades"). "Frieden" literally means "peace"—in this case, they believed that peace could be achieved only once the "oppressors on Terra Firma" were eliminated.
March–June: The Jovian Moons Campaign began. Jovian secessionist attacks on United Nations Colonial Advisors on the moon Io led to three months of fighting between the Earth military and Jovian "Frieden" forces. Though this was not the first armed conflict in our Solar System, it was easily one of the bloodiest, and is generally considered to be the spark of increased friction and militarization that followed.
The Jovian Moons Campaign escalated tensions, as Earth's national governments—many of which sponsored colonies within the system—began fighting proxy wars off-planet. As these proxy wars continued, tensions on Earth mounted, leading to a number of armed conflicts on Earth itself.
The Rain Forest Wars: Armed conflict ripped through South America, as Koslovic, Frieden, and UN forces all clashed over ideological differences, sparking additional conflicts off-planet.
December: In a conflict that stemmed from the Rain Forest Wars, the three primary Earth factions clashed again, this time on Mars. A series of lightning strikes against Koslovic forces near the Argyre Planitia marked the first extra-terrestrial deployment of Marines. The campaign was an unqualified success. As a result, future military doctrine favored large contingents of Marines for ground assaults and ship-boarding actions.
2164: Interplanetary War
UN-sponsored military forces began a pattern of massive buildups, culminating in the first real interplanetary war. After the successful Marine deployment on Mars, recruitment drives and propaganda tactics strongly bolstered UNSC (United Nations Space Command) forces. UN forces defeated Koslovic and Frieden forces on Earth, then began a systematic and dedicated drive to crush their remnants on the various planets they held throughout the system. At the conclusion of these brushfire conflicts, Frieden and Koslovic forces were defeated in the face of a massive, unified and very powerful UN military.
A unified Earth government was formed in the wake of these conflicts. Now, the victors were forced to deal with a less obvious but equally serious threat: overpopulation and a massive military that had no enemy to fight.
In the postwar period there were massive population surges and the overpopulation, coupled with the destruction and famine bred by the Rain Forest Wars, threatened to destabilize the economy.
2291: Faster than light
A team of researchers, physicists, and mathematicians working in secret developed the Shaw-Fujikawa Translight Engine, a practical means of propelling spacecraft across vast interstellar distances. This new engine allowed ships to tunnel into "the Slipstream" (also called "Slipspace"). Slipspace is a domain with alternate physical laws, allowing faster-than-light travel without relativistic side-effects. Faster-than-light travel is not instantaneous; "short" jumps routinely take up to two months, and "long" jumps can last six months or more.
The SFTE generated a resonance field, which when coupled with the unusual physics of the Slipstream, allowed for dramatically shorter transit times between stars; however, scientists noted an odd "flexibility" to temporal flow while inside the Slipstream. Though no human scientist is sure why travel time between stars is not constant, many theorize that there are "eddies" or "currents" within the Slipstream—there is generally a five to ten percent variance in travel times between stars. This temporal inconsistency has given military tacticians and strategists fits—hampering many coordinated attacks.
2310: First Wave
The Earth government unveiled the first in a line of colony ships—and volunteers were in great supply. Conditions on Earth had deteriorated in the face of overpopulation, so hitching a ride out to a colony became an attractive option.
Each colony ship was assigned military personnel and escort ships to help better utilize the massive (and expensive) standing fleets that, in the collapse of armed dissent, were soaking up a great deal of funding and resources.
Because FTL travel in this period was still fairly new and expensive, colonists and military personnel faced a stringent regimen of physical and mental testing. In theory, only the best-qualified citizens and soldiers were allowed to colonize "nearby" worlds. This was the birth of the Inner Colonies.
2362: The Odyssey
The Odyssey launched on January 1, 2362. The lead ship in a fleet of colony vessels, the Odyssey—laden with troops and terraforming gear—spearheaded the colonization of a new world. This sparked the first wave of human expansion beyond the confines of the Solar System.
2390: Inner Colonies
By 2390, the colonization of the Inner Colonies was fully underway. There were 210 human-occupied worlds in various stages of terraforming, and the population burden across human-controlled space was largely stabilized.
2490: The Birth of the Outer Colonies
Expansion continued at a rapid pace, and by 2490 human space encompassed more than 800 worlds throughout the Orion Arm of the Milky Way (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.
2525: The Covenant War begins
On April 20, 2525, contact with the Outer Colony, Harvest, was lost. After failing to re-establish contact with Harvest, the Colonial Military Administration sent a scout ship, the Argo, to investigate. Contact with the Argo was subsequently lost after the ship arrived in the Harvest system.
The CMA dispatched a battle group to Harvest, consisting of three warships. The lead ship in the battle group, the Heracles, returned to Reach, badly damaged and with heavy casualties. The ship's commander reported that an alien warship with powerful weaponry was present, and had decimated Harvest, exterminating the colonists (and presumably destroying the Argo).
The battle group engaged the alien warship and was thoroughly routed. The Heracles—following the destruction of the other two ships in the group—jumped out of the system, but due to the damage she had sustained, took several weeks to make it back to Reach.
The Earth military immediately upgraded their alert status and began preparations to move in and retake Harvest. By December, a massive Earth war fleet under Vice Admiral Preston Cole mobilized, one of the fastest such mobilizations of such a large fleet in the history of humankind.
2525: The Battle of Harvest
Cole's war fleet engaged 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—learned that a number of outlying colonies had been destroyed, leaving behind no survivors. Cole began to move his fleet around in an attempt to intercept the invaders. Ground and ship-to-ship battles began in earnest, raging throughout the Outer Colonies. During one ground engagement, human forces captured one of the aliens. Before he succumbed to his wounds, interrogators learned that the aliens referred to themselves as "the Covenant."
The Outer Colonies massacres
Over the next several years, Cole's forces were hammered, despite his excellent leadership and tactical brilliance. It was simply a matter of being outgunned; Covenant kill ratios tend to top four to one in ship-to-ship combat.
By November of 2535, virtually all of the Outer Colony worlds had been destroyed by the Covenant. The "Cole Protocol" was established by military order: All human vessels must ensure that Covenant forces do not find Earth. Human ships—when forced to withdraw—must not travel on an Earth-bound vector, even if that requires jumping into Slipspace without proper navigational calculations. If blind jumps are not possible, the ship's captain must order self-destruction if capture is imminent. In addition, it is also imperative that the powerful ship A.I. data cores not fall into Covenant hands, so part of this protocol involved either the removal or destruction of the ship's A.I. in extreme situations.
2536-2552: The Siege of the Inner Colonies
Covenant forces swarmed into the Inner Colonies. For several years the war fell into a pattern: humans emerged victorious from isolated battles—typically during ground operations—but at a horrible cost. In space combat, the humans lost at a precipitous rate, and one by one, the colonies fell.
Covenant forces arrived at Reach and obliterated Earth's last major military stronghold. The battered cruiser, the Pillar of Autumn, fled the devastation, carrying with it the sole surviving SPARTAN. The SPARTANs—an elite unit of supercommandoes, equipped with the fearsome MJOLNIR assault armor—were created as the ultimate soldiers.
Now, only one SPARTAN remains to carry the fight to the enemy. In accordance with the Cole Protocol, Captain Jacob Keyes—the Autumn's commanding officer—plotted a random, long-distance jump, hoping to lure the Covenant fleet away from Earth.
After deactivating the drive, the Autumn dropped into a distant, uncharted system. There was a Covenant fleet in the system as well, near a planet-sized ring-shaped construct, "Halo."
Halo's secrets must be unlocked if humanity is to survive the Covenant onslaught...