Reach was the second most important human settlement behind Earth, both for being a highly-populated and vibrant community, and for being one of the largest military complexes the UNSC had. Reach was also home to the Spartan-II program, where Doctor Catherine Halsey and SCPO Franklin Mendez kidnapped and trained children into an elite team of super-soldiers.
By 2552, the war against the Covenant was not going well, and enemy fleets were pushing farther into humanity's inner colonies, quickly encroaching on what little space people called their own. The situation got far worse in July, when the Covenant found Reach, beginning a covert invasion of the planet before a full-scale attack commenced a few weeks later. Though Reach was heavily defended, including over 300 million military personnel, the planet fell to the Covenant, leaving nothing between them and Earth.