 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 . One of the races affected was humanity , who came into conflict with the Forerunners as a result . Exhausted by their war and after having explored all other options , the Forerunners conceived a weapon of last resort to combat the Flood , called the Halo Array . 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 sponsors the Spartan @-@ II program to create an elite group of enhanced supersoldiers , whose purpose is to suppress the rebellion 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 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 . 
