After decades of fighting with the San'Shyuum, the Sangheili accepted the fact that they were forced to adapt and study their Forerunner relics, or face defeat by the San'Shyuum. The Sangheili reluctantly use the technology to advance their armor, weapons, and warships.
The Sangheili-San'Shyuum War ends with a peace treaty. They form a religious covenant simply called the Covenant. The Writ of Union is the peace treaty that combines the two. It later integrates others into the Covenant.
The Sangheili became the warriors and guardians for the San'Shyuum Prophets. Together, they search for and collect Forerunner artifacts for religious gain. Their main goal is to walk the Great Journey which can only be achieved by activating the mythical Halo Array.