War of Beginnings

The War of Beginnings was the first inter-species war between the Sangheili and San'Shyuum, and paved way for the epoch of the Covenant.

The state of the Sangheili sphere
The War of Many Banners was a large, multi-power Sangheili civil conflict. Much of what took place in that war set the stage for the Sangheili going into the War of Beginnings a century later. The defeat of the Thar'shi Sphere by the Crimson League resulted in numerous changes. The Thar'shi began focusing more on new settlements rather than conquering pre-existing territories and had spent numerous resources rebuilding their devastated military.

Out of their new colonization drive came the world of Ulgethon. On the military side, they had become the most standardized military force.

Ulgethon & the periphery
The San'Shyuum's first contact with the Sangheili occurred on the underdeveloped fringe colony of Rerto'tha, where the Reformists were welcomed as gods or messengers thereof (the details are unclear and possibly mythologized), emboldening them to approach the leaders of the better-developed Ulgethon. Ideological differences regarding the treatment of Forerunner artifacts ensued; most pointedly, the Sangheili of Ulgethon were outraged at what they saw as a grave blasphemy in the San'Shyuum having "stolen" a divine vessel, and in so doing assumed the Forerunners' place. This resulted in the local Sangheili summarily executing the San'Shyuum envoys and declaring holy war on the other species. The War of Beginnings had begun, and would soon envelop much of the Sangheili sphere. The massacre at Ulgethon led to the first major battle between the Sangheili and the San'Shyuum, and a protracted siege of the world.

Thar'shi forces and the San'Shyuum would come to blows over Forerunner artifacts on the planet of Divine Equilibrium. Led by Ussa 'Xellus, son of the famed Reta 'Xellus, the Thar'shi forces unfortunately couldn't overcome the San'Shyuum but they overall denied them access to those artifacts.

Sangheili alliance
Although the war only affected the Thar'shi Sphere at first, other groups would soon be enveloped by the fighting as word spread and the San'Shyuum struck at Sangheili worlds regardless of their affiliation. Quite soon, however, the Reformists learned how factionalized the Sangheili were, and sought to exploit this. Learning everything they could about the Sangheili, the San'Shyuum began approaching various independent groups and offering them benefits in exchange for allegiance, an offer some groups accepted. Meanwhile, the major Sangheili groups realized the severity of the threat to their religion and way of life, and a grand council was convened between the leaders of the major Sangheili factions. Even as the specifics of their religious beliefs differed, most still accepted the universal sanctity of Forerunner relics. The war was no longer about politics or territory but the protection of the relics they all held most dear- a holy war. For the first time, sworn enemies (most prominently the Thar'shi Sphere and the Crimson League) would join forces. This marked the beginning of the Righteous Host, the largest alliance of Sangheili in history up until that point.

Striking the heart
Over the course of the war, the San'Shyuum periodically attempted to find allies among different Sangheili groups, though most of the major factions remained against them. For much of the battling, the relatively progressive Irshun League sought to remain neutral. However, seeing the potential to damage, and get ahead of their geopolitical rivals, the Irshun League decided to side with the San'Shyuum. Their proximity to the other Sangheili powers allowed San'Shyuum to forces to strike far deeper into the other Sangheili factions.

Exodus
Many in the Thar'shi Sphere and Crimson League could not stand to be under this new Covenant, prompting numerous desertions and disappearances. A prominent figure to do so was Ussa 'Xellus who would flee known space with several allies, founding a splinter civilization outside the Covenant's reach.