After the emergence of the Flood on ancient human worlds around 107,445 BCE, human civilization was desperate to expand their territory away from Flood-controlled regions of space. As part of this expansion, humans clashed with Forerunner-populated worlds, sparking a war between the two species. Not knowing that humanity was running from a threat to their existence, the Forerunners retaliated fiercely, eventually beating humanity back to only a handful of worlds, including their capital at Charum Hakkor over the next one thousand years. Though humans were allied with the San’shyuum for most of the war, the San’shyuum were eventually convinced to cease their support in return for leniency from the Forerunners. This betrayal, along with the attrition of a millennia of fighting the Forerunners and Flood, tipped the scales firmly in the Forerunner’s favor, securing a defeat for humanity. Though humanity fought valiantly to the end, they were eventually overrun at the Battle of Charum Hakkor and the final few worlds they still inhabited.
The end of the war nearly saw the extinction of the entire human race, though it was the Librarian, having only recently learned of the Flood and the threat it posed to the entire galaxy, who convinced those in power to spare the race in hopes of developing a cure to the Flood. Instead of eliminating all of humanity, the remaining species were devolved into lesser beings, then placed on worlds such as Erde-Tyrene, the Halos, and the Lesser Ark, where they were allowed to live a pre-industrial life in exchange for the Forerunners studying them for a cure to the Flood parasite.