Milky Way galaxy

The Milky Way is the galaxy where the Humans and the Covenant reside. It is a barred spiral galaxy that lies with the Local Group of galaxies neighborhood of the Universe, visible from Earth as a band of light in the night sky. Although the Milky Way is one of billions 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 place of origin for humanity, 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, or if they expanded further out through the 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 to two Halo Installations and the teleporter to the Ark.

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.

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.