In 2525, humanity made first contact with intelligent extraterrestrials. An alien religious alliance known as the Covenant. Our attempt at peace was met by plasma fire. The place of first contact, our colony of Harvest, was glassed, it's surface scorched with plasma until the oceans and atmosphere boiled away and the soil was vitrified. To the rest of humanity, contact with Harvest was lost. battlegroups were sent to find the cause and reestablish contact. Only one ship returned. It brought news of an alien ship with Harvest destroyed and a message in perfect English: "Your destruction is the will of the Gods, and we are their instrument." With an alien alliance with superior technology declaring war against humanity with genocidal intent, humanity's greatest Admiral was called out of retirement to lead a fleet of 80 ships just to destroy one. Upon arriving, the fleet suffered heavy losses. But Admiral Cole was able to coordinate the fleet to fire on the ship at the same time, breaching it's energy shields, and destroying the ship. The war would drag on for 27 years, and humanity would be burned from every world the Covenant found. The war followed a predictably depressing pattern, while the UNSC (United Nations Space Command; the United Earth Government's military branch) could contend with the Covenant on the terrestrial battlefield, the one in space, wasn't so even sided. Covenant warships were more advanced than UNSC ships in every regard. In order for it to be considered an even fight, the UNSC would need to outnumber a Covenant fleet by 3 to 1. By 2552, the UEG's outer colonies had been considered destoryed by the Covenant. While some Outer Colonies did survive, they were in Covenant controlled space, and couldn't be helped. The Inner Colonies were now being picked off and suffering heavy losses. However, a plan was created to end the war. Operation: RED FLAG. Utilizing the remaining SPARTAN-IIs, humanity's greatest soldiers, an AI advanced beyond any other (Cortana), and a warship (the Pillar of Autumn) outfitted with advanced tech. As Operation: RED FLAG was prepared for at Reach, humanity's fortress among the stars, our military hub world, a Covenant fleet of over 300 ships arrived in system. Operation: RED FLAG was scuttled by the captain of the Pillar of Autumn, and it was redirected to aid in the defense of Reach. Ultimately, Reach's defense grid fleet of over 150 ships and 20 orbital Super Magnetic Accelerator Cannons would be destroyed, and only a single Spartan, the Master Chief Petty Officer John 117, would make it back aboard the Autumn before it fled Reach. As it fled Reach, a Covenant armada would follow it. With Covenant ships faster than UNSC ones, the armada would beat the Autumn to it's destination. When the Autumn finally reached it's destination, it found a massive artifact of unknown origin, a giant ring world with the inside of it habitable, Halo. Not wanting to damage Halo, the Covenant instead sent boarding craft. To prevent the Covenant from gaining valuable UNSC knowledge, the Master Chief was tasked with getting Cortana off the ship and protect her, while the Captain Keyes would crash lang the ship on the ring. That's it for now, and by the way, this is the short version I'm giving you. Meaning I skipped a couple thousand years of history on purpose, and am leaving out other details.