The Human-Covenant War was a major conflict pitting the Unified Earth Government and its United Nations Space Command against the Covenant Empire. There were three additional factions sporadically involved in the conflict; the Flood, a small number of surviving Forerunner constructs, and an unlikely ally to Humanity - the Reapers. These factions proved to be invaluable to the efforts of humanity, as the information taken from Forerunner constructs and the Flood invasion of High Charity were helpful in splitting the Covenant apart.
The war was started by the Covenant whose Prophets made a discovery which led them to believe that Humans were directly related to the Forerunners, contradicting their ancient religion. Seeking to keep various political and religious orders intact, a war of genocide was declared against the human race. Over the next few decades the Covenant glassed and destroyed many worlds due to their technological and tactical superiority over the Humans, however, the Covenant did not realise the destructive capability of the Human-allied Reapers, as whilst this campaign of genocide was taking place, the Reapers were quietly aiding Humanity by destroying outskirt Covenant worlds. The Covenant invasion eventually led to the Battle of Reach, which led to the story of Jorgey and Johnny beginning.
The conflict was also the cause of the Great Schism within the Covenant. The Elites decided that Truth was bullshitting and realised they were never going to win anyway, so they split and joined the Human-Reaper forces. Forced to fight a war on two fronts, the formerly monolithic empire was torn apart. The conflict lasted for a total of twenty-eight years.