The Master Chief, the Halo series' main protagonist, was abducted at the age of six, to become part of the military program known as the SPARTAN II project. He lost his last name, and gained a number, becoming John-117, although later he was known as either Spartan-117 or "the Master Chief."

The Spartans were trained and surgically augmentated when they were young to become a team of super-soldiers to use against Human rebellions to minimize collateral damage. The project began with 75 children, but 30 died in surgery and 12 became too crippled by it to be of any use in combat.

When the Covenant began a holy war against humanity, the Spartans (combined with the powerful MJOLNIR exoskeleton armor) were put to use against the Covenant. Only about four Spartan II's were alive by the end of Halo 2, although all of the killed Spartans are officially listed as MIA to uphold the illusion that Spartans are unkillable. Spartan 117 stood out among the other recruits not because of any exceptional physical or tactical skill but rather because of his great leadership abilities and his death-defying luck.