The Covenant is an alien military hegemony consisting of many species in the Milky Way Galaxy. These races include creatures known as San'Shyuum, Sangheili, Jiralhanae, Unggoy, Kig-Yar, Mgalekgolo, Yanme'e, No one is quite sure when they were founded, but it is theorized that their empire began construction before the time of the Messiah on Earth.
The Covenant follows a religion of worship of ancient aliens known as Forerunners, who left behind ancient and powerful technology that the aliens manipulated and converted for their own purposes. The religious and political aspect of the Covenant is led by the San'Shyuum, or Prophets. The Military is led by the Sangheili, or Elites. The other races serve military roles.
During the time of Blue Team, the Covenant attacked Earth three times. Their initial invasion, The War of Trespass, marked the first time in Earth's history that the planet was invaded. It threw much of the world into chaos, but eventually, the invaders were repelled by Blue Team. A few years later, the Covenant struck an alliance with the radical and maniacal tyrant Boss Cass , who ended up working for Malefor and forming Malefor's Holy Empire. During this time, the Covenant became nearly subservient to Cass, who held them in an inescapable deal of servitude through the use of his enforcers, the Bears.
Multiple years later, the Covenant attacked a third time, this time under the leadership of Sangheili radical and warlord, Jul 'Mdama. This third invasion was even more destructive than the first, and led to an age of strife on planet Earth. Eventually however, the Covenant was repelled once again, thanks to the Faceless empire and Blue Team's combined efforts. Jul 'Mdama was slain, and the Covenant retreated once again, so far remaining far away from Earth.