Arcadia was a human outer colony, originally colonized in 2429. Arcadia quickly became a thriving colony due to its good climate, growing to a population of almost three-million by the 2500s. In 2531, the Covenant attacked Arcadia, and while the UNSC forces led by Admiral Preston Cole managed to fight off the Covenant, most major population centers were destroyed, and most of the population either died or left the planet.
The colony remained untouched by the Covenant for the next fifteen years, and slowly colonists brave enough to risk another Covenant attack returned to repopulate the planet. This proved disastrous, however, as the Covenant returned in 2549 to finish the job they started, glassing the planet and killing all life on the surface.