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 and destroyed the majority of the Inner and Outer Colonies. It was also significant for bringing about the end of the 3,400-year-old Covenant, leaving in its wake hundreds if not thousands of successor states embroiled in war with each other.

The conflict officially began when the triad of newly-ascended High Prophets declared humanity as an affront to their gods and must be exterminated. For the vast majority of the conflict, the Covenant was an overwhelming aggressor, where their advanced technology and sheer numbers allowed them to route the UNSC 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 through 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 generally 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 March 1, 2553, officially bringing an end to over 27 years of 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, 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 known, 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, destroying them once they had served their purpose. 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 Prophets' 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.

The goal would ultimately change towards outright protecting Forerunner artifacts, as humanity made concerted attempts to destroy relics than allow them to fall into the Covenant's hands. This was crucial towards continuing the war until the species was exterminated, inspiring a second wave of dogma among their troops.

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.

Contradictions
As the Human-Covenant War continued to draw on, many military officers would begin to criticize the Covenant's policy of staying in the war, particularly among the Sangheili. This was due to a number of reasons, largely thanks to a culture shift that occurred as zealous veterans were replaced with inexperienced recruits, many of which who were too young to remember a time when the Covenant wasn't in some state of war. Other supporters were hardcore conservatives who held the principles of honor just as highly as the Great Journey itself.

The largest support for an end to the war, and to offer humanity membership into the Covenant, was their combat prowess. After the initial period of chaos had been passed, the excellent skill of their commanders, the superior command structure they operated, and their fighting spirit allowed them to inflict massive casualties against invading legions. Heroic actions against impossible odds earned the respect of the Sangheili field commanders, which inspired several to treat their human foes much like they would treat other Sangheili opponents, such as giving unprepared garrisons the chance to equip themselves before attacking. It was clear that they were not the cowards they had been expecting, but equals in many ways.

In addition, their conflicts were often compared to that experienced during the Taming of the Hunters. The Lekgolo also committed grave sins by destroying Forerunner artifacts, yet they were forgiven and inducted as a client species because of how well they defeated the Covenant's armies. The story of the Lekgolo's assimilation would be a well-known tale of the Covenant's mercy and forgiveness, and the Sangheili would begin questioning what made the humans so different as to warrant the drawn-out war. The fact that the Hierarchs repeatedly rejected these beliefs led to suspicions that there was more to the war than was immediately apparent, like the San'Shyuum were attempting to hide something that the other species did not know about.

Another major development was the capture of humanity's AIs. While these were undoubtedly far more superior than anything the Covenant had ever designed, it was clear from their programming and capabilities that they lagged far behind what ancilla were capable of, and were most likely indigenous creations. While nevertheless sacrilegious, the crime was not as damning as it first appeared. Late in the war, the Covenant would secretly begin to experiment and even field modified versions of these machines for their purposes. While restrained in capabilities and now fully devout to their cause, it caused an uproar among the few conservative officers who were aware of their existence.

Final year
The final year of the war would see some of the most climactic battles of the war.