The SPARTAN-II project is the successor to the SPARTAN-I project, known as Project ORION. The SPARTAN-Is were named after the Spartans of Greece. Sparta was a military-oriented state; from birth, its children were screened for disabilities or defects, and those that failed the checks were left to die. Its population was mostly slaves (helots), allowing the city-state's actual citizens to spend every waking moment training for war.
Historically, the Spartans won many wars simply by arriving to fight. Appreciative of this reputation, the UNSC decided to use the name for their grand project. The result would be a group of supersoldiers gifted with inhuman abilities. With this goal in mind, Dr. Catherine Elizabeth Halsey set out to gather children who were genetically suitable for the project. They found a total number of 150 trainees, but for budget reasons only half that number were "conscripted," a process that involved kidnapping the child in question and replacing him or her with a flash clone. These clones were unstable (due to the process used to clone them not being perfected) and quickly died of what appeared to be natural causes. They were officially conscripted by Section Three of the Office of Naval Intelligence, which is why the player's character (the Master Chief) holds a Naval rank and is not a member of the Marine Corps, despite typically fighting alongside Marines.The group was taken to the UNSC's space colony Reach, where from the age of 6 they were trained in all aspects of combat and tactics, as well as learning various other academic information in lieu of attending a proper school. They also were administered many physical and mental augmentations. The process was not painless, nor was it entirely safe, but Dr. Halsey knew she would have to sacrifice a few for the good of many. The result of the project was a group known as the Spartans, nearly invincible warriors with superior skills and tactical prowess. - http://en.wikipedia.org/wiki/SPARTAN-II_Project