Mantle

"The Mantle. You still hold to that [belief] after all that has happened? After this thing has consumed a million worlds? Can't you see? Belief in the Mantle sealed our doom! Weakened our [protectorates], bred dependence and sloth. Our [so-called Guardianship] has stripped those we would keep safe of any capacity for self-defense! Were we such noble [Guardians] when we drew our line and abandoned billions to the parasite?"

- Librarian

The Mantle was a Forerunner philosophy. While no precise definition of it exists, it is heavily implied that it was the source of both the Forerunners' "authority" and their apparent duty to preserve the galaxy's biological diversity. Bridging the social strata between an interstellar Marshall Plan and a religious, but benevolent, stewardship, the Forerunners took responsibility for the protection and cultivation of the species and planetary systems within their domain.

History
Forerunner mythology held that the Mantle was passed down to them by an even earlier and more advanced race, the Precursors; to what extent this was true, or even whether or not it was a common belief, remains unknown. Regardless of its origin, the Forerunners seem to have handed their Mantle down to humanity; it may play a significant part in humanity's Reclaimer status.

Some Forerunners doubted the purpose and validity of the Mantle, dismissing it as superstition and fairy tales. The Librarian, in particular, had such doubts, and even went so far as to hold it responsible for the downfall of the Forerunners at the hands of the Flood. The then-rampant AI Mendicant Bias believed that the Forerunners used the Mantle to obstruct any change, even that which might eventually be beneficial.

Their central government was entirely dictated by the Mantle, as were the religious beliefs of individual Forerunners. However, inspiring peace throughout their domain by minimizing their military capacity and disarming their weapons cache proved to be a costly approach for the Forerunners when the new alien threat emerged.

Rather than allowing every species to rise on their own merits, the Forerunners, following their Mantle, gave them no need to do so, serving as protectors against any threats that might have forced technological or cultural progression. An unforeseen side-effect was that only the Forerunners were able to hold their own against the Flood (at least once they rearmed), while their charges were unable to defend themselves; the Forerunners had unintentionally left the galaxy vulnerable to Flood infection.