Human-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 warships 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 out-gunned; 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 about 516 planets in the inner colonies with Reach as one of their few remaining major strongholds. 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 almost 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 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; fighting is also known to have occurred in North America, with engagements being depicted near Havana, Cuba in Ghosts of Onyx. It is unknown what or how many UNSC forces remain on Earth's other continents or how many colonies and ships remain intact at this point.

United Nations Space Command
Leaders: HIGHCOM

Homeworld: Earth

Military Forces: 220,000,000 (2530) 15,000 Star ships

The Covenant
Leaders: The High Council

Homeworld: High Charity

Military Forces: Exact numbers are unknown. Nearly 400 million (2530), and 60,000 Star ships

2525

 * The UNSC makes first contact with the Covenant at the UNSC colony world Harvest. The Covenant destroy the colony without warning or provocation.
 * Spartan IIs help win the very minor Battle of Chi Ceti.

2530

 * The Covenant destroys the UNSC colony world of Eridanus II.

2531

 * Battle of Harvest. The UNSC and Covenant meet in ship-to-ship combat for the first time; the Covenant are outnumbered and outgunned, nevertheless the battle is a close victory for the Humans.

2535

 * The Covenant destroys the UNSC colony world of Jericho VII in the Battle of Jericho VII.
 * In the course of four short years, UNSC forces have been hammered, despite excellent leadership and tactical brilliance. It is simply a matter of being outgunned; Covenant kill ratios tend to top three to one in ship-to-ship combat. By 2535, virtually all of the Outer Colony worlds have been destroyed by the Covenant.

2536

 * Covenant forces begin to swarm into the Inner Colonies. For several years the war falls into a pattern: humans win isolated battles but only when they outnumber the covenant 2-3 to 1 in space and 3-5 in ground combat, able to hold their own on the ground, but at a horrible cost. On the ground the Covenant inflict superior kill ratios of 3-5 to 1 due to their superior training and military. In space combat, the humans are losing at a precipitous rate, and one by one, the colonies fall.

2537

 * The Battle of New Harmony
 * The Battle of New Constantinople
 * The Siege of the Atlas Moons
 * Operation: PROMETHEUS: Spartan-III Alpha Company is killed by Covenant forces

2544

 * The Battle of Miridem

2545

 * Operation: TORPEDO: Spartan-III Beta Company is killed by Covenant forces

2549

 * The Siege of Paris IV

2552
July: August: September: October: November
 * Battle of Sigma Octanus IV in the Sigma Octanus System. It is the UNSC's single greatest victory since Harvest. The Covenant, however, gain what they are looking for: the location of Installation 04. They also determine the location of Reach.
 * Battle of Reach. In the largest naval battle yet in the war, 314 Covenant ships destroy 152 UNSC ships and 20 MAC gun platforms around Reach, and win a major victory, destroying the UNSC's last great military stronghold, besides Earth.
 * The Battle of Installation 04. A single UNSC ship, the UNSC Pillar of Autumn escapes to Installation 04. After four days of hard fighting, the Covenant discover the Flood organism and Forerunner constructs, hampering their efforts to control the installation. The melee ends with Installation 04 destroyed with only a handful of survivors.
 * Operation: FIRST STRIKE. Six SPARTAN-II supersoldiers are able to rout and destroy a fleet of 500 Covenant starships preparing to jump to Earth.
 * The First Battle of Earth. Though a relatively small and lopsided battle; 15 Covenant ships fighting 75 UNSC vessels and 300 MAC platforms, the event is significant in that the Covenant discover the location of Earth. The UNSC wins the fight.
 * The Battle of Installation 05. A single Covenant ship survives the battle of Earth and retreats to Installation 05. It is pursued by a single UNSC ship, the UNSC In Amber Clad. Again, a massive fight ensues between the UNSC, Covenant, Flood, and Forerunner Constructs. The Flood appear to win control of the Installation.
 * The Civil War of the Covenant begins. Covenant forces are thoroughly divided between the Elites and the Brutes. Every race in the Covenant takes sides.
 * The Second Battle of Earth. The Brute faction of the Covenant returns to Earth in force, apparently conquering Africa.
 * The Battle of Onyx
 * Civil War of the Covanent
 * The Second Battle of Earth