The Mantle of Responsibility is a philosophical concept created by the Precursor civilization millions of years ago. The Precursors, hyper-advanced begins responsible for creating life in the Milky Way galaxy if not the universe, crafted the concept of the Mantle to serve as the guiding principles for the species who would succeed them as guardians of all life in the galaxy. However, when the prospective Forerunners fell out of favor with the Precursors and the Mantle was said to be given to humanity, the Forerunners rebelled and exterminated the Precursors where they could be found and drove them to the outer fringes of the galaxy. With the Precursors gone, for the moment, the Forerunners self-appointed themselves holders of the Mantle and reigned over the galaxy for generations.
Under the Forerunners, the Mantle of Responsibility was invoked to justify the Forerunners actions. Like the Marshall Plan enacted after World War II, the Forerunners would help aid the species who accepted its benefits. However, species who accepted the Forerunners control and species who did not ultimately ended up the same. Without the need to expand their borders and lacking the ingenuity to improve their lives, the technological advancement of several species degenerated. The exceptions to this rule were the San 'Shyuum and humanity, both of whom would later challenge Forerunner authority and the Mantle. The human-Forerunner War erupted as humanity laid siege to countless worlds both Forerunner and Forerunner client species. Several Forerunners, including the Librarian and Master Builder, invoked the Mantle to retaliate against the humans because, as they argued, their actions threatened the stability of the galaxy and Forerunner authority. Another motive for the retaliation can be found in the human belief that they were the true inheritors of the Mantle and had served as ideological enemies to the Forerunner for years, the war provided the Forerunners an opportunity to rid a rival. The Didact, the highest-ranking Promethean Warrior-Servant, cautioned a more reasoned response to appeal to more civil course of action according to the Mantle. He argued for attack, but was able to persuade the Council that exterminating humanity would be an even greater threat to the concepts of the Mantle than humanity was currently posing.
After the war, the Mantle was further twisted to suit the beliefs in a rapidly corrupting Forerunner government. The discovery of the Flood threat, the true cause of humanities aggressive expansion, brought the Warrior-Servant and Builder rates against each other. The Didact proposed the Shield Worlds as a way to shelter the Forerunner civilization and other species against the Flood, this stood against the Master Builder’s Halos, devices that could wipe out all life in the galaxy. The Didact saw the Halos as counter-productive to the Mantle’s mission statement of guarding the galaxy and all of its inhabitants and it amounted to little more than genocide. The Master Builder was able to argue that his Halos would ensure Forerunner supremacy and appealed to the Builder-friendly Forerunner Council, sending the Didact into exile in the process.
However, the Flood threat ultimately brought the Forerunner’s interpretation of the Mantle crashing down to failure. Thousands of years of protection and aid under the Mantle rendered the galaxies species technological inept and left them as easy prey to the Flood as it spread throughout the galaxy. After the rouge AI known as Mendicant Bias destroyed the Council and several of the 12 Halo rings, it justified its actions by claiming that the Mantle turned the Forerunners into little more than dictators and rendered the galaxy weak under their care. Faith in the Mantle began to wane during the war, the Librarian herself began to see it as a silly fairy tale that heralded the downfall of the Forerunners. Yet the Didact, released from his exile, still clung to the Mantle, though his experiences with a Gravemind warped his sense of morality. He would argue that the Mantle entitled the Forerunners as rulers of the galaxy and would use this to justify the use of a Composer, a machine that turns to organic beings into digital ones, on humans being kept on a Halo ring to serve as his Promethean Knights.
The Didact’s actions were seen as a gross violation of the Mantle, he had let his prejudice and anger overwhelm him and destroy innocent lives in the process. He was wounded by his wife on the Shield World of Requiem and contained in a Cryptum in the hopes he would meditate on his actions and learn to help humanity, the new holders of the Mantle as far as the remaining Forerunners were concerned. 100,000 years after the Halos firing, humanity is now on the cusp of reclaiming the Mantle for themselves. However, they must contend with the responsibility of protecting the galaxy while still dealing with effects of a post-Human-Covenant War galaxy. Will they rise above their anger and grief of the war and help the species who had tried to exterminate them or will they follow the path of the Forerunners that doomed the galaxy to stagnation and near eradication by the Flood?