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. [1]

2520

Spirit of Fire Refit:

The UNSC Spirit of Fire undergoes a series of refits in this year that transform it from a colony ship into a warship. Refits include the addition of a Magnetic Accelerator Cannon to the ship.[4]

2525

February 3, Harvest:

The Harvest orbital platform made long range radar contact with an object. It was made of an unknown material. Contact with Harvest was lost shortly thereafter. [2]

2525

April 20, The Argo:

Sent by the Colonial Military Administration to investigate the loss of contact with Harvest, the scout ship Argo arrived in-system on 20 April. Other than a brief transmission to confirm their exit Slipstream position no further reports were made. [2]

2525

October 7, Contact:

Fleet Command assembled a battlegroup to investigate and they enter the Harvest system on 7 October.  The battlegroup consisted of the destroyer Heracles, commanded by Captain Veredi, as well as the frigates Arabia and Vostok.

They discovered drastic changes to the planet Harvest’s surface and no sign of the three million colonists.  An alien ship was detected in orbit.  The ship did not attack until Captain Veredi tried to initiate communications.  They responded by broadcasting the following message in the clear, in our own language: “Your destruction is the will of the Gods…and we are their instrument.”

The Vostok and Arabia were lost with all hands while the Heracles jumped out of the system, but took several weeks to return to Reach due to damage sustained in the battle. [2]

2525

November 1, War:

The UNSC is placed on full alert.  Vice Admiral Preston Cole is mobilizing the largest fleet action in human history to retake the Harvest System and confront this new threat. 
 
The Spartan II program is accelerated to help deal with the new Covenant threat. [2]

2531

The Battle of Harvest:

Admiral 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. During one ground engagement, Human forces capture one of the aliens. Before succumbing to his wounds, interrogators learn that the aliens refer to themselves as "the Covenant" (roughly translated). [1]

2535

The Outer Colony Massacres:

Over the next four 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 AI data cores not fall into Covenant hands, so part of this protocol involved either the removal or destruction of the ship's AI in extreme situations. [1]

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. [1]

2552

Halo:

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." [1]