War of Beginnings

The Sangheili-San 'Shyuum War was an interstellar war set between the Sangheili and the San 'Shyuum races, over religious and technological differences. At the conclusion of the war, the Writ of Union was penned and initiated, and the Covenant was kindled.

Discovery
After leaving their homeworld in 2100 B.C.E., the San 'Shyuum encountered the Sangheili in 938 B.C.E., who also had experience with Forerunner technology. The San 'Shyuum believed that incorporating Forerunner technology was allowed by their religion. To the Sangheili, such a thing would have been unthinkable heresy.

War
During the combat, the Sangheili had an early advantage, with each warrior comparable in skill to several San 'Shyuum in battle. However, the San 'Shyuum's technological superiority, and possession of the Forerunner Dreadnought, meant that they had the advantage in space battles, using the Dreadnought to great effect in devastating hit-and-run strikes against the Sangheilian War Fleets. Eventually in 876 B.C.E., in order to ensure the survival of their race, the Sangheili were forced to amend their position of the treatment of Forerunner technologies and began studying their relics and incorporating elements into their warship, weapon and armor designs. In 852 B.C.E. the two races signed a treaty known as the Writ of Union to form the early Covenant with the San 'Shyuum as the head religious leaders and the Sangheili as the physical backbone.