The SPARTAN-IIs' secret to being seemingly invincible and matchless in combat has always been their training, which made them capable of wearing the MJOLNIR armor, and made them superior in strength, speed, accuracy, reaction times and intelligence to all other UNSC military personnel.
Between 2511 and 2517, 150 five- to six-year-old children were selected from near perfect psychological and genetic markers that were set in Dr. Halsey's selection protocols. The children were screened for certain genetic markers: strength, agility, even predispositions for aggression and intellect. But due to lack of facilities and funding, only 75 children were chosen for the program. The other half of the 150 original candidates were "weeded out" possibly through "chance" and "luck," the single flip of a coin.
Between August 17 and September 23, 2517, the accepted candidates were taken from their homes in the middle of the night, injected with a sedative, and rushed through Slipspace to Reach, where they were going to be trained. They were only recently brought out of cryosleep when Dr. Halsey told them that they would be indoctrinated into the UNSCDF to defend Earth and all her colonies, and that they could not return to their parents, that there would be a great deal of hardship on the road ahead, encouragement, and various other things. The children were then escorted to barracks where they had a meal, and went to bed at around 2330 hours.
September 24, 2517
The Spartans were rudely awakened early in the morning by CPO Franklin Mendez and his instructors at 0530 hours. Mendez ordered them to go to the showers and those who hesitated were hit with an Electric Baton as "encouragement." After that they went outside and Chief Petty Officer Mendez ran them through 100 reps of jumping jacks, sit ups, deep squats, knee bends, and leg lifts.
Afterward, Mendez and the instructors jogged them about a mile to Déjà, a "dumb" AI that taught them a wide variety of lessons, including mathematics, science and history between physical training. Their introductory lesson was a video of the original Greek Spartans fighting against Persian soldiers at Thermopylae. After the lesson concluded, they jogged two miles to the "Playground," a maze in mid-air with a bell at the top, where they were divided into teams of three. The last team to reach the bell at the end would not get supper, as an incentive to win. Though John-117 reached the bell first, his other teammates were last, forcing them all to do without supper.
September 25, 2517
The Spartans were woken, took their showers and did their morning calisthenics and ran a mile to Déjà. Their next lesson was about a pack of wolves hunting a moose, which began to teach the SPARTAN's about teamwork. After the lesson they ran the two miles to the playground, which had been changed and was much more complicated. This was also when CPO Mendez noticed SPARTAN-087, SPARTAN-034, and SPARTAN-117's teamwork.
September 26, 2517 - July 11, 2519
In this time Mendez taught them to string a trip line, fire a rifle, and treat a chest wound. Mendez's exercises often had a twist to them such as stun mines on the obstacle course, snipers with paint guns during parade drills, and others. They were taught to be strong in body and mind.
July 12, 2519
Pieces of paper were distributed, fragments of a map of the local region that had been shredded. They were set down alone to navigate to the marked extraction point and picked up there by an Albatross dropship. They were told that the last one to the extraction point would be left behind and have to walk back.
Instead, however, the Spartans combined their strengths to meet up together at a lake near to the LZ. They discovered an Albatross dropship, as promised, but the occupants were not in uniform and carried live-ammunition weapons. The Spartans lured the men into an ambush, either knocking them out or killing them, and hijacked the Albatross, using Deja to pilot it back to base. John made sure he was last to board the pelican to make sure that nobody else would have to walk home.
John was made team leader for his planning of the Spartans reaction to the training operation.
July 14, 2523
For even more serious training, the Spartan-II's were matched against the formidable Tango Company in simulated wargames. Still only 12 years old at the time, the Spartans were nevertheless able to continuously beat Tango Company. Eventually, the men of the company resorted to brutality as a form of payback, with "accidents" occurring with Spartans captured in the wargames. Eventually, Tango Company's CO, furious at his men's inability to stop the Spartans, ordered them to use live rounds instead of stun rounds.
September 11, 2525
The newly augmented Spartans were transferred to an abandoned Titanium mine on Reach to enhance secrecy and reduce the injury risk to other personnel. Due to the uneven strength, speed, intelligence and accuracy of the Spartans, their trainers were forced to don inadequate powered armor simply to keep up with them. Training missions invariably ended in a victory for the Spartans, with even security cameras neutralized. It is unknown whether the trainers were members of Tango Company or not.
- Halo: The Fall of Reach.