The Halo franchise takes place in a science fiction universe, where there are four major factions players encounter or control. Halo s story has 26th century humanity, led by the United Nations Space Command (UNSC), caught in a war with a genocidal alien coalition known as the Covenant. In the video game Halo: Combat Evolved, a lone UNSC ship,The Pillar of Autumn, which is surrounded by Covenant discovers a mysterious ringworld known as "Halo". The massive installation was built by an enigmatic race known as the , who have long since disappeared for unknown reasons; the Covenant worship the Forerunner as gods. During the course of the game, players discover that the Halos were built by the Forerunner as a weapon of last resort against the Flood, a extragalactic parasite which is driven to consume all sentient life. The Forerunner were forced to activate the Halo network, killing themselves and any potential Flood hosts, in an effort to starve the Flood to death. In Halo: Contact Harvest, it is revealed that the Covenant leadership discovered that humanity was in fact descended from the Forerunner, and decide to commit genocide against humanity in order to suppress the truth. The Flood, meanwhile, escape the confines of Halo and threaten to spread across the galaxy once again.
A large portion of the series' success lies in the creation of a believable world, and Bungie reinforced the fictional factions the player interacts with via a lengthy design process with the release of each game. The overall design of each faction was slowly developed before the release of the first game in the series, Halo: Combat Evolved, and continually refined in the later games' development.
The United Nations Space Command is the main faction of future humanity, spanning multiple worlds besides Earth. Formed in the 2160s, during a period when remnants of old cultural ideologies clashed over supremacy in the Solar System , the UNSC primarily served as an overseer of United Nations military operations in space. After initiating massive militarization propaganda throughout its off-world colonies, the UN, through the UNSC, defeated neo-communist and neo-fascist forces in a conflict generally known as the "Interplanetary War", consisting of several side-battles that took place on Mars, the Jovian Moons and the South American rain forests. Although the war brought a great deal of suffering to both the colonial population and the residents of Earth, it also united humanity's military forces into a common armed force by the end of the 22nd century: the UNSC.
Before the Covenant attack on Harvest, humanity was in turmoil, with the remote colonies fighting for independence from the UNSC. To help quell the revolts, the UNSC commissioned the SPARTAN Project, which created elite super-soldiers called Spartans. The job of these 'Spartans' was to stealthily eliminate Insurrectionists, who were revolting against the UNSC and the Inner Colonies. When the Covenant began decimating the outer colonies, these Spartans became humanity's best hope against the technological superiority of the Covenant. By Halo: Combat Evolved, Earth—whose location is kept secret from the Covenant—is one of the last remaining Human bastions; it falls under attack during Halo 2. The end of the Human-Covenant war only comes after one of the Spartans, MCPO John-117, also known as the Master Chief, the game's main character, helps to eliminate the Covenant's leadership and destroy the Flood.
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