Human-Covenant War

"Your destruction is the will of the Gods... and we are their instrument."

- The Covenant's declaration of war.

The Human-Covenant War, also known as the Covenant War and the Great War by humanity, was a major interstellar conflict between the United Nations Space Command and the Covenant Empire, with smaller engagements involving the Flood and surviving Forerunner constructs. The Covenant War is rightfully regarded as the single-most destructive conflict in humanity's history, where it claimed an estimated 28 billion human lives, destroyed the majority of the Inner and Outer Colonies, and inflicted significant infrastructural damage on the Sol system. It was also significant for bringing about the end of the Covenant's 3,400-year-old history, leaving in its wake hundreds if not thousands of successor states embroiled in war with each other.

The conflict officially when a triad of newly-ascended High Prophets declared humanity as an affront to their gods and must be exterminated. First shots were fired on Harvest on February 11, 2525, when UNSC militia forces were attacked by the crew of the missionary ship. For the vast majority of the conflict, the Covenant would maintain itself as the overwhelming aggressor, where their advanced technology and sheer force of numbers allowed them to win the vast majority of victories in both space and on the ground. Within ten years, nearly all UNSC-controlled Outer Colonies were lost. This trend continued at a slower pace within the Inner Colonies, although they were ultimately successful at taking even the strongholds of Paris IV and Reach. By contrast, humanity would only succeed in scattered engagements, and any attacks behind the secured alien lines were rare and short-lived.

The War was won by humanity almost solely due to the beginning of the Great Schism in late-2552. A series of massive losses and unprecedented reforms led to a civil war between the Sangheili Schismatics and Jiralhanae-dominated Loyalist forces. A not-insignificant fleet under the command of Thel 'Vadam allied themselves with the UNSC, and the combined force was successful at defeating the Covenant over the Ark, preventing the activation of the Halos and decapitating the Covenant's leadership in one stroke. Although hostilities would continue afterwards, there was a general sense that the conflict had come to a close. A ceasefire between the majority of the Sangheili Schismatics operating within the human sphere and the UNSC was ratified as a treaty on February 24, 2553, officially bringing an end to the conflict.

The conflict dramatically changed the political landscape of the Orion Arm. In the region once occupied by the former Covenant, hundreds if not thousands of individual polities resided in its place, with several being superior to even the pre-Covenant War UNSC in terms of territory, economic output, and especially military might. Many of these were quickly destroyed or absorbed into their larger neighbors. The Concord of Reconciliation, a coalition founded by the Schismatics, quickly rose to become the dominant known power in the Orion Spur. Meanwhile, the UNSC quickly came to terms with the fact that it was no longer the single-largest military force know, undertaking several large-scale reforms as it attempted to reclaim its former glory.

Casus Belli
Officially, the newly-ascended High Prophets of Truth, Mercy, and Regret justified the Covenant's war by convincing their military commanders and population that humanity openly defiled the Forerunner legacy. In front of a grand tribunal, Hierarchs explained that humans had openly looted artifacts from every reliquary they discovered, rather than worship and carefully excavate such devices. Far worse, it was argued that their mass-use of AIs, already a cardinal sin in the eyes of the Covenant, were actually Forerunner ancilla stolen from their installations and brainwashed to suit their needs.

To support this, the Hierarchs presented evidence based on recent events. The luminaries on Covenant starships had identified certain humans with a particularly obscure and rare glyph, which had traditionally been thought to mean 'reclamation.' Truth argued that this interpretation was wrong, and the computers were actually noting that the tagged individuals had directly participated in the desecration of reliquaries. Secondly, humans had made liberal use of AIs, which was already a grievous sin in the eyes of devout followers. It was reasoned that the only way they could have gotten so many was if they were Forerunner ancilla had been taken from their installations and reprogrammed to serve their needs. Finally, they stated that the unexpected departure of the Keyship Anodyne Spirit was due to the rage of the Oracle aboard when it was told what they knew of humanity, an act that would have killed billions on High Charity. It was only due to the new High Prophet's pleas that it calmed down. This last piece of evidence was the strongest one, as even those that still doubted the Hierarchs could not answer what prompted the Oracle's reaction.

However, the real reason of the war was quietly buried by the three Prophets. It bestowed them with knowledge that threatened the very unity and faith of the Covenant, which had been founded with the belief that all the races of the galaxy were equally considered the inheritors of the Forerunner's legacy. The only way to prevent civil war was to exterminate any trace of humanity.