The SPARTAN-II program was the first major success of the SPARTAN programs, an effort to produce elite soldiers through mechanical and biological augmentation, being the first in the series of programs to meld advanced exoskeleton technology with the soldiers' superior physiques. Spartan-IIs experienced the most extensive and invasive physical augmentations out of all the Spartan generations, starting augmentation at early pubescent ages increased their physical mass and height to around seven feet by adult age.
After being kidnapped and sedated at 6 years old, the Spartan-II candidates were taken to the colony world Reach, where they began their training under Chief Petty Officer Franklin Mendez and the AI Déjà, who also acted as an aide for Halsey on the program. To marginalize the civilian lives they had once led, their names became a combination of their given names and a number, with family names being discarded. Based mainly within the Reach FLEETCOM Military Complex, the Spartans endured a great deal of hardship during their first years of training: they were placed into situations and drills that pushed their abilities to their very limit and beyond. Their hard training was complemented with high-level education, which included mathematics, science, history, reading, writing, and military tactics.
Although his training methodology was tough, even brutal in some regards, Chief Petty Officer Mendez always instilled discipline, honor, and respect into the Spartans. He taught the Spartans how to kill, but at the same time he taught them the difference between right and wrong. During this time, John-117 rose to be the de facto leader of the Spartans. Mendez trained the Spartans until 2525 when, at the age of fourteen, they would go through the toughest part of their training: the biological augmentation procedures, which would kill 30 of the 75 conscripted children and cripple twelve others who would "wash out" of the SPARTAN-II program, going on to join the Office of Naval Intelligence. Only 33 survived the procedures without physiological deformities, of whom two would commit suicide in the following weeks upon encountering their flash clones. The bodies of those candidates who "washed out" were placed in cryonic suspension in the hope that they could some day be resuscitated. At least some of the candidates who were crippled may have been physically rehabilitated in later years.
While many outsiders view them as single-minded, the Spartan-IIs remain independent thinkers; from early on, Dr. Halsey's intent was to mold the candidates into effective soldiers through "persuasion and acclimation" as opposed to the brainwashing suggested by some of her colleagues within ONI, believing that in order to be truly efficient, the Spartans must be able to properly understand and embrace the significance of their mission. As such, the Spartans possess a profound understanding of their role as weapons and the weight of the life imposed on them. Most of them do not hold grudges over their abduction and treatment, instead having come to embrace their lives as Spartans as both a necessity and something they were born to do.
The Spartan-IIs' training emphasized dedication to one's duty as well as winning at any cost, values most of the active Spartan-IIs remain committed to. They have frequently been shown to ignore concerns over their personal health as long as there remains a mission to complete[53] and tend to become incredibly frustrated when prevented from acting in a combat situation; because of this they generally prefer planetary operations to being based aboard starships with no way to contribute to an engagement.
The Spartan-IIs enjoy a great deal of solidarity and camaraderie within their group due to them having grown up together; indeed, they consider their fellow Spartans family, something that was encouraged in their indoctrination from a young age. Having been taught to refrain from overt emotional displays, the Spartan-IIs most commonly convey their emotions through subtle body language and cryptic gestures. They possess numerous unique internal habits and signals conveyed through simple gestures or statements, making interactions among their group largely indecipherable to outsiders. Only a select few outsiders, including Dr. Halsey and Déjà, are privy to some of the best-held secrets within the group. Their familiarity with each other's body language allows the Spartan-IIs to function in perfect synergy in combat, their teamwork abilities having been described as borderline telepathic. Although the Spartan-IIs often have difficulty socializing with baseline humans outside situations of a strictly military nature, they are rather gregarious among themselves and habitually cluster together while out of the battlefield, commonly engaging in activities such as playing cards, exercising together, reading, or maintaining their personal equipment.
Most humans view the Spartan-IIs with awe, sometimes even revulsion, due to both their reputation and intimidating presence, the latter complemented by the seemingly unnatural speed, precision and grace of their movements. Few outsiders are willing to as much as touch a Spartan, such as shaking one's hand.
The Mjolnir armor was created in parallel to the SPARTAN-II project and is the most technologically advanced piece of hardware in human hands. It can provide a Spartan with an incredible amount of protection and strength. The onboard computer system relays basic tactical information including IFF tags, a motion tracker, weaponry information, and health and energy shield level readouts. Later models of the Mjolnir system are capable of carrying a starship-grade AI which can provide tactical data in the field.
A specialized version of the neural interface is implanted into the skull of a Spartan-II. The standard neural interface provides the neural link between the Spartan-II and the Mjolnir armor. This device allows a starship-grade AI to be carried "between" the host's brain and later models of the Mjolnir armor.