In the distant past, an intellectual race called the Forerunners fought an alien parasite known as the Flood. The Flood, which spread through infestation of sentient life, overran much of the Milky Way Galaxy. One of the races affected was humanity, who went into space and came into conflict with the Forerunners. Exhausted by their war and after having explored all other options, the Forerunners conceived a weapon of last resort to combat the Flood. Using an installation known as the Ark, they built large ring-shaped megastructures known as Halos. The Halo Array, when activated, would destroy all sentient life in the galaxy—depriving the Flood of their food, after which samples of the lost lifeforms would be reseeded. Delaying as long as they could, the Forerunners activated the rings and disappeared. Tens of thousands of years later, in the 26th century, humanity under the auspices of the United Nations Space Command, or UNSC colonize many worlds thanks to the development of faster-than-light "slipspace" travel. Tensions escalate between the older and more stable "Inner Colonies" and the remote "Outer Colonies", leading to civil war. The UNSC creates an elite group of enhanced supersoldiers, known as Spartans, to suppress the rebellion covertly. In the year 2525, the human colony of Harvest falls under attack by a theocratic alliance of alien races known as the Covenant. The Covenant leadership declares humanity heretics and an affront to their gods (the Forerunners) and begins a holy war of genocide. The Covenant's superior technology and numbers prove decisive advantages though effective, the Spartans are too few in number to turn the tide of battle in humanity's favor.