Milky Way

The Milky Way is the galaxy where the Humans (in real life also) and the Covenant reside, and is approximately 100,000 light years in diameter. It is a barred spiral galaxy that lies within the Local Group of galaxies neighborhood, which is just a very small portion of the Universe. Visible from Earth as a band of light in the night sky, although the Milky Way is just one of many of galaxies in the observable universe, the Galaxy has special significance to Humanity as it is the home galaxy of the planet Earth.

Orion Arm
The Orion Arm is the origin for humanity and their home planet Earth, with at least 800 worlds colonized by the UNSC at one point. It is also the area of origin for the various races of the Covenant, with possibly even more worlds. It is assumed to be the place of origin for the Forerunners as well, but their mysterious nature and advanced technology means that it is not known exactly where they came from, if they expanded further out through the galaxy, or even if they originated from outside of this galaxy.

The Orion Arm is the setting for the Human-Covenant War of the 26th Century and the far more ancient Forerunner-Flood War, witnessing at least two massive wars. It is also the home of at least two Halo Installations and the teleporter to the Ark.

Forerunner-Flood War
Prior to approximately the year 97,448 BC (Human years), the previous galactic community was dominated by the Forerunners, a mysterious and now extinct species that possessed extremely advanced technology and an empire that spanned a significant portion of the galaxy. However, encounters with a parasitic sentient species, the Flood, led to the Forerunner-Flood War, with the Flood consuming or killing the great majority of the galaxy's sentient species as the Forerunners first studied them, and then tried to stop them. Eventually, the Forerunners were forced to construct the ultimate weapon, the Halo Array, in order to stop them - at the same time, killing its builders. The Forerunners built safe havens, including the Shield Worlds and the Ark, to save the remaining sentient species, before activating it, scouring the galaxy of all sentient lifeforms and killing the Flood except for samples kept on the Halos, then releasing the saved species from their havens to their home worlds.

Human-Covenant War
Almost 100,000 years afterward, the galaxy had recovered substantially, leading to the rise of two great civilizations - the UNSC, comprised of the majority of humanity and controlled from Earth, and the Covenant, a hegemony of many different species who worshiped the Forerunners as gods. Contact between the UNSC and Covenant was not peaceful, leading to the Human-Covenant War, which destroyed hundreds of human colonies and killed billions of colonists and UNSC military personnel. The discovery of one of the Forerunners Halo installations set forth a series of events that would not only end the war, but also destroy the Covenant - the Covenant Civil War, or Great Schism.

Great Schism
Triggered by the replacement of the Elites with the Brutes, the Elites left the Covenant, in the process that wrestled a significant amount of the Covenant's populace and resources, creating the Separatist faction. The Separatists, realizing that the Prophets were conducting an unjustified war against humanity and planning a misguided activation of the Halo Array, allied with the UNSC to stop them, leading to the Battle of Installation 00. Here the Separatists crushed the remainder of the Covenant Loyalists and ended the war.

Solar System
The colonization of the Milky Way includes the early colonization attempts of the UN and the UNSC, beginning with the colonization of the Solar System during the 2100's. The first planet believed to be colonized is Mars, followed by the Jovian Moons. During 2160 - 2164, the Solar System was the site of the Interplanetary War, the most brutal conflict known to man since World War II. The war was fought between the UN naval and infantry forces, the Neo-Communist Koslovics (led by hardliner Vladimir Koslov), and the Frieden rebels (Jovian secessionists). Following the successful Marine deployments on Mars during the Argyre Planitia Campaign of 2163, the UN ultimately defeated both Koslovic and Frieden rebels throughout the Solar System, thus forming the United Nations Space Command.

Beyond
The development of the Shaw-Fujikawa Translight Engine in 2291 enabled the UNSC to explore and colonize planets beyond the confines of the Solar System, thus the Inner Colonies were born. By 2310, the first line of colony ships was made public by the Earth Government. On January 1, 2362, the Odyssey, the first colony ship capable of faster-than-light travel was launched. It was laden with troops and terraforming gear. This sparked the first wave of colonization in the inner colonies. By 2390, the colonization of the Inner Colonies was underway. There were 210 human-occupied worlds in various stages of terraforming, and the population burden across human-controlled space was largely stabilized.

Colonial expansion continued and by 2490, UNSC colonial space encompassed over 800 worlds, large and small, in the Orion Arm. The success of colonization of the Outer Colonies made the Inner Colonies the primary stronghold, both military and economic, 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.

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, plunging humanity into a deadly war with an alien race known as the "Covenant". Harvest was glassed from orbit, and marked the beginning of the war with the Covenant. The war was named the Human-Covenant War. Since the war's beginning, hundreds of human colonies have been destroyed and billions of civilian and military personnel have been killed by the Covenant. It is unknown how many colonies remain at the end of the war, although remarks made by Lord Hood shortly before the Ark was destroyed suggested heavily that Earth was the last human-controlled planet, especially when he mention:"What we should do Commander, is understand, clearly, that this is humanity's final stand, here, at Earth. We go, we risk everything; every last man, woman, and child. If we stand our ground, we might just have a chance." and:"Earth...is all we have left. You trust Cortana that much?".

Covenant
Because the Covenant have been space-faring for thousands of years, they have explored a significantly larger portion of the galaxy than humanity has. Based primarily in the Orion Arm of the Milky Way, it is unknown if they maintain territories outside of the region, but there are several homeworlds and colonies located in the Orion Arm.

The Human-Covenant War has devastated numerous human worlds, but at least one Covenant world, Joyous Exultation's, population was wiped out by Admiral Whitcomb repurposing the NOVA Bomb in retribution for the destruction of Reach. It is unknown if the Separatists and Loyalists destroyed worlds under the control of the other faction, but both Te and Balaho were almost glassed during the Covenant's history.

Places

 * Perseus Arm
 * Norma and Cygnus Arm
 * Scutum-Crux Arm
 * Sagittarius Arm

The Milky Way, and the Orion Arm in particular, has a large number of inhabited or, at least, habitable, planets within it, with the United Earth Government and the Covenant Empire both maintaining distinct territories of their own.
 * Orion Arm

Human Systems
 * 111 Tauri System
 * Victoria
 * Chi Ceti System
 * Chi Ceti IV
 * Cygnus System
 * New Jerusalem
 * Epsilon Eridani System
 * Reach
 * Epsilon Eridanus System
 * Tribute
 * Eridanus II
 * Eridanus Secundus
 * Epsilon Indi System
 * Harvest
 * Groombridge 34 System
 * Hellespont System
 * Troy
 * Hydra System
 * Sigma Octanus System
 * Sigma Octanus IV
 * Sol System
 * Earth
 * Mars
 * Jovian Moons
 * Zeta Doradus System
 * Onyx

Human Colonies From Unknown Systems
 * Arcadia
 * Ballast
 * Beta Gabriel
 * Biko
 * Charybdis IX
 * Coral
 * Draco III
 * Emerald Cove
 * Far Isle
 * Gilgamesh
 * Harmony
 * Jericho VII
 * Mariposa
 * Minister
 * Miridem
 * New Harmony
 * Paris IV
 * Tantalus

Covenant Systems
 * Tala System
 * Balaho
 * Oth Sonin System
 * Doisac
 * Y'Deio System
 * Eayn
 * Napret System
 * Palamok
 * Svir System
 * Te
 * Urs System
 * Sanghelios
 * Tau Ceti System
 * 51 Pegasi System
 * 51 Pegasi-B System
 * Pegasi Delta
 * Hawking System
 * Lambda Serpentis System
 * Salia System
 * Joyous Exultation

'''Forerunner/Flood Planets
 * DM-3-1123
 * LP 656-38
 * CE-10-2164-d
 * Substance
 * Threshold
 * Iris Nebula

Trivia

 * The only known engagement or conflict to ever take place "outside" the Milky Way (considered the Outer Rim) is the Battle of Installation 00.