Covenant War

On October 7, 2525, A UNSC battle group consisting of the UNSC Heracles, UNSC Arabia and UNSC Vostok entered the Harvest system and discovered 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 and SPARTANs 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.