War of Beginnings

The War of Beginnings was the first inter-species war between the Sangheili and San'Shyuum, and was 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. 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 made disastrous first contact with the Sangheili on Ulgethon resulting in a protracted siege of the world.

Thar'Shi forces and the San'Shyuum would come to blows over Forerunner artifacts on the planet of Glorious Balance. 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.

Striking the heart
For much of the battling the Irshun League was 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 both of 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.