On Killing by LtCol Dave Grossman, 2009, Excerpts
Leaders with legitimate, societally sanctioned authority have greater influence on their soldiers; and legitimate, lawful demands are more likely to be obeyed than illegal or unanticipated demands. Gang leaders and mercenary commanders have to carefully work around their shortcomings in this area, but military officers have tremendous potential to cause their soldiers to overcome individual resistance and reluctance in combat.
The Roman formation had mobile, highly trained, and carefully selected leaders whose primary job was not to kill but to stand behind their men and demand they kill. The Roman centurion was a professional leader who had the respect of his soldiers because he had come up through the ranks and had previously demonstrated his ability in combat. This kind of legitimacy is completely different from that associated with leadership in civilian life, and the Greek leader was primarily a civilian whose peacetime legitimacy was not easily transferred to the battlefield.