The SPARTAN Programs refer to a series of United Nations Space Command projects designed to create physically, genetically, technologically, and mentally superior super soldiers as special fighting units. Although the word "SPARTAN" is written in all capital letters when used in reference to the programs in a formal context, it is not an acronym; the super soldiers are known simply as "Spartans".
The basic concept for the SPARTAN programs has been around since the late 22nd century when the first bioengineering protocols were developed for the Interplanetary War. Since the establishment of these protocols humans have been using performance-enhancing equipment and augmentations to make them stronger and faster than previously possible. Spartans have been deployed numerous times throughout human conflict, leading up to and during the Human-Covenant War. The notion of creating super soldiers first arose as a plan to enhance normal human soldiers into powerfully augmented special operations commandos. Initially the three separate programs were humanity's various attempts to create the perfect soldiers, in order to patrol the colonies, protect civilian and government populations, crush uprisings in their infancy and ensure others don't rise up as a result.
The Spartan soldiers of the UNSC are named after the Spartans of Ancient Greece, specifically during the time of the Persian invasion led by Xerxes I. The wars between invading Persians and defending Greeks would be known as the Greco-Persian Wars, and lasted from 499-448 BCE. During this war, the Battle of Thermopylae occurred, in which 300 Spartan soldiers leading 5000 other Greeks under the command of Leonidas I, held back a significantly larger force of Persian soldiers. All of the Spartans were eventually killed, but they successfully killed 20,000 Persians (according to Herodotus), and bought the Greeks time to prepare a better defense for Greece. The Spartans' formidable resilience resulted from their rigorous training in the agoge, which started when they reached the age of 7.