The Milky Way is the galaxy where the humans 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, the Milky Way is just one of many galaxies in the observable universe, though the Milky Way has special significance to humanity as it is the home galaxy of its planet, Earth. The Milky Way is orbited by two small satellite galaxies known as the Magellanic Clouds.
The Milky Way is a barred spiral galaxy of either SBb or SBc classification on the Hubble Sequence. It has two major arms, each sporting several minor arms that stem off of the major arm. One of these major arms, the Carina and Sagittarius Arm, is home to the Orion Arm. Splotches of intense star birth and formation can be seen in the arms. These areas are pink/red in hue and appear as wisps or clouds situated in the galactic plane. These are easily seen on any level featuring the Milky Way in the sky (Isolation, The Ark, etc.).
 The Orion Arm
- Main article: Orion Arm
The Orion Arm is the origin of humanity and their home planet Earth, with at least 800 worlds inside human territory at one point. It is also the area of origin for the various races of the Covenant, with possibly even more worlds. The Forerunner homeworld Ghibalb is also located in the Orion Arm.
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 slipspace portal to the Ark.
 Other Galactic components
The Milky Way is composed of a disk, in which all the arms and the majority of galactic matter lies, and the galactic bulge, essentially the center of a galaxy. The bulge is home to the galactic nucleus, the dead center of a galaxy and home to an extremely dense clustering of ancient stars. The nucleus is also thought to support a supermassive black hole. Additionally, most known galaxies possess a halo of old and dying star clusters left over from the galaxy's formation. These clusters are situated up to 100,000 light years from the galactic bulge. Habitable planets may certainly lie within the halo, and some clusters may escape the range of the Halo Effect given their distance from the galactic center.
100,000 years before the present day, the previous galactic community was dominated by the Forerunners, a now extinct species that possessed extremely advanced technology and an empire that spanned a significant portion of the galaxy. Millions of years before the rise of the Forerunners, an even more advanced civilization known as the Precursors controlled the Milky Way. Having reached the pinnacle of technological advancement, they were believed to have significantly contributed to the evolution of life across the galaxy, even created several species, such as Forerunners and humans, sometimes wiping their creations out as they saw fit. Millions of years before the emergence of the Flood, the Precursors chose to destroy the Forerunners. Before this could happen, however, the Forerunners rose up against their creators instead, and seemingly ended their rule in the galaxy.
Approximately 150,000 BCE, the Forerunners, originating from Ghibalb in the Orion complex, rose to preeminence in the galaxy. Around this time, humanity, still composed of many species, arose from their homeworld Earth, known as "Erde-Tyrene" at the time. They mastered interstellar travel and began to move their civilization along the Orion Arm of the Milky Way. They later allied with the San 'Shyuum species, forming a prosperous interstellar empire based on the planet Charum Hakkor. Around 110,000 BCE, humanity and their San 'Shyuum allies came into contact with the Flood, a sentient parasite which was said to have arrived from the Magellanic Clouds. Hundreds of systems were lost to the parasite in the resulting conflict. Meanwhile, humanity's aggressive expansion and the perceived violations of the Mantle prompted the Forerunners to take action and begin a war against humanity. Despite being forced to fight a two-front war against the Flood and the Forerunners, humanity eventually drove the Flood out of the galaxy. However, the Forerunners eventually emerged victorious over humanity.
Following the war in approximately 109,000 BCE, the Forerunners dismantled humanity's civilization, regressing them into a Tier 7 species. The remaining humans exiled to their homeworld, where the supreme Forerunner Lifeshaper, the Librarian, established a research station. In the following millennia, the Librarian tended to the humans, ensuring that their ancient species made full recovery and eventually allowing their civilization to begin developing anew.
 Activation of the Halo Array
- Main article: Forerunner-Flood war
10,000 years after humanity's conflict with the Flood, the parasite resurfaced and attacked worlds of the Forerunners' domain. Encounters with 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 activate 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 in plan known as the Conservation Measure, before activating the Array, scouring the galaxy of all sentient life forms and killing the Flood except for samples kept on the Halos, then releasing the saved species from their havens to their home worlds.
 Galactic recovery
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, since the Covenant possessed more advanced technology, mostly reverse-engineered from Forerunner artifacts.
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 and begin a Covenant civil war, or Great Schism. Triggered by the replacement of the Elites with the Brutes in the Covenant military hierarchy, the Elites left the Covenant, in the process that consumed a significant amount of the Covenant's populace and resources, creating a 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 destruction of the Covenant in the Battle of Installation 00.
Following the end of the Human-Covenant War, humanity began a long process of rebuilding, while at the same time making an effort to ensure their former enemies - mainly the Sangheili - would not rise again. Meanwhile, humanity gained significant advances in technology due to the discovery of Forerunner artifacts during and after the war.
- Main article: UNSC
 Solar System
The colonization of the Milky Way includes the early colonization attempts of the United Nations and the UNSC, beginning with the colonization of the Solar System during the late 21st and early to mid-22nd centuries. During this time, Luna, Mars, the Jovian Moons and a number of asteroids in the system were colonized by Earth's various national governments, under the direction of the UN. During 2160 - 2170, the Solar System was the site of several conflicts, the most prominent being the Interplanetary War, the most brutal conflict known to man since World War II. The war was fought between the UN, the neo-communist Koslovics (led by hardliner Vladimir Koslov), and the Frieden rebels (Jovian secessionists). Following the successful UN-commissioned Marine deployments on Mars during the Argyre Planitia Campaign of 2163, the UN ultimately defeated both Koslovic and Frieden rebels throughout the Solar System. These conflicts also marked the beginning of the United Nations Space Command.
The development of the Shaw-Fujikawa Translight Engine in 2291 allowed humanity to leave the confines of its home star system for the first time and explore the wider galaxy, ushering in an age of rapid colonization and expansion. The first line of colony ships were unveiled in 2310, and on January 1st, 2362, the Odyssey, the first colony ship capable of faster-than-light travel, was launched laden with troops and terraforming gear. Over the next 28 years, the UNSC began terraforming and colonizing more than 210 worlds, which would become the Inner Colonies.
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 termed the Human-Covenant War, and was the most destructive conflict in the galaxy's history since the Forerunner-Flood war 100,000 year prior. 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.
At its peak, the UNSC encompassed a number of star systems, with hundreds of planets, moons and planetoids settled by colonists, some terraformed to greater degrees than others depending on the local conditions and presence of indigenous life. The Inner Colonies encompassed a region of space in at least a 10.5 light-year three-dimensional radius from Earth. Beyond that were the Outer Colonies. Other colonies were established even further - Coral, at 42 light-years; and Madrigal, which lay in the 23 Librae system, 83.7 light-years from Earth. In 2456, Harvest was the furthest Outer Colony, at six weeks of Slipspace travel away from Madrigal and two months from Reach. It is unknown how many colonies remain at the end of the war in 2553, although a number of Inner and Outer Colonies are known to have survived.
- Main article: Covenant
Because the Covenant have been space-faring for at least a thousand years, and possess far superior slipstream navigation technology to humanity, 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. Additionally, the Covenant have discovered and explored several Forerunner Shield Worlds, similar to one destroyed by human forces, from which they have acquired much of their advanced technology. Exactly how many of these worlds they have found, and their status, is unknown.
The Human-Covenant War has devastated numerous human worlds, but at least one Covenant world, Joyous Exultation, was devastated by Admiral Whitcomb's 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, the Lekgolo and Unggoy homeworlds respectively, were almost glassed during the Covenant's history.
 Orion Arm
- Main article: Orion 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 Unified Earth Government and the Covenant both maintaining distinct territories of their own. Other planets and systems were once home to the now-extinct Forerunners, whose interstellar empire spanned the galaxy until their disappearance.
- The only known engagement or conflict to ever take place "outside" the Milky Way is the Battle of Installation 00.
- The Milky Way can be seen from The Ark.