"In the distant past, a powerful 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. Exhausting all other strategies, the Forerunners conceived the Halo Array—ring-shaped megastructures and weapons of last resort that would destroy all sentient life in the galaxy to stop the Flood. Delaying as long as they could, the Forerunners activated the rings and disappeared. Nearly a hundred thousand 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 between the government and colonies desiring independence sparks violent clashes. The UNSC sponsors the SPARTAN-II Project to create an elite group of enhanced supersoldiers, whose purpose is to suppress the rebellions covertly. In the year 2525, human worlds come 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 begin a genocidal holy war." "The rediscovery of the Halo rings sets the humans against the Covenant, who believe they are instruments of transcendence, not destruction. Master Chief and his artificial intelligence Cortana are instrumental in the destruction of a Halo ring to stop the Covenant and the threat of the Flood. The Covenant descends into civil war over the revelation their religion is false, and the disgraced former Covenant commander Arbiter and his race, the Sangheili, help the humans stop the Covenant leadership from activating the Halo Array. The Human-Covenant War ends, though conflict continues to rage throughout the universe. The UNSC creates a new generation of Spartans, and tensions between the UNSC and colonists resume."
Halo's Wikipedia page, accessed February 12, 2020