First Great War

The First Great War (known alternatively as Great War I, the Human-Covenant War, or the War of Vain Attrition) was an interstellar, multi-decade war beginning in 2525 and closing with the Great Schism's end in 2554. It took the lives of more than one hundred billion sentient beings; as such, it would be the most brutal conflict for any of the participating species at the time of its happening.
The war began as Imperial-sponsored privateers stumbled upon a Human outpost world by the name of Harvest; before long, what was originally a botched negotiations conference erupted into a full fledged war between three loosely aligned parties: the UNSC-CAA conglomerate, the Covenant Empire, and a myriad of unaffiliated insurgent groups, though the latter would hardly be considered as a friendly alliance but rather an assimilation of similarly minded, belligerent individuals.