Nobody can doubt the effectiveness of the SPARTAN-II and SPARTAN-III Programs, both in terms of military performance and as psychological deterrents and morale sappers. Among the Covenant, who treat all such creatures with a nearly supernatural regard, they acquired a reputation as Demons, and were the only warfighters to ever give the Sangheili, a warrior race that prides itself on its fearlessness, reason to hesitate in their battle plans against humanity. Their existence single-handedly allowed humanity to survive the Human-Covenant War, holding back the Covenant onslaught long enough for the Great Schism to begin, shattering it forever and giving humanity respite.