In Every Army There is a Mob Waiting to Escape
It is common to assume that battles involving masses of men engaged in close combat were horrifically bloody; in fact, they were rarely so, at least as long as the units remained engaged. When the infantry was arrayed in close phalanx formation, only the first two ranks could actively engage in any fighting, and then not for very long. It has been estimated that the lines of the phalanx could remain engaged in actual combat for less than thirty minutes before exhaustion began to take its toll. Men in the front ranks would be quite fortunate to remain in contact for half that time before being overcome by exhaustion. Moreover, until the introduction of the Marian reforms in the Roman army (100 [B.C.E.]), no army had learned the technique of having its front ranks break contact, withdraw in good order through the other ranks, and be replaced with a fresh line of infantry. For the most part, stamina governed the tempo of the battle….
As long as the men within the phalanx held their ground and remained together, it was difficult for any significant killing to occur. Even the cavalry could not be decisive against infantry formations that held their ground. Cavalry charges were inherently unstable to begin with due to the absence of the stirrup, and horses would not throw themselves against a packed wall of humanity, especially if the spears of the formation were raised against them. Yet, in every army there is a mob waiting to escape, and its motivation is fear. The real killer on the ancient battlefield was fear. Men in combat have their instinctive flight or fight responses held in delicate balance by the thin string of intellect. Continued stress increases the probability that someone within the ranks will lose his nerve and run. Sometimes the actions of a single soldier are sufficient to forge the onset of panic in an entire unit. Once the integrity of the formation began to erode, the ancient soldier was at very great risk of death or injury.
— From Sumer To Rome, pp. 83-84