Some more info re extreme mental stress experienced by warriors during battle, taken from this
Cannae battle article, using works by Marshall and Keegan for reference --
The warriors' deep dark secrets -- '[D]uring combat the soldier may become so gripped by fear that most of his thought is directed toward escape. But if he serving among men whom he has known for a long period or whose judgment of him counts for any reason, he still will strive to hide his terror from them ... The majority are unwilling to take extraordinary risks and do not aspire to a hero's role, but they are equally unwilling that they should be considered the least worthy among those present [SLA Marshall, 1947, p. 149]'
Pelters' peer pressure -- After the battle lines would have been drawn, skirmishers will try to disrupt the enemy's formation by getting near them to launch volleys of missiles. 'Many frightened troops [i.e., skirmishers] would have used their weapon in these circumstances out of a desire not to appear weak in front of their friends [the most important reason why men fight in battle, per Marshall (1947) and John Keegan (1976)] ... a large proportion of these would have fired blindly, without even attempting to aim.'
Not much change after 2,000 years -- In comparison, Marshall (1947) in his study of the American infantry's performance in WW2 observed that on average, 'no more than 15% of men ever used their weapon in any given engagement. This figure rose to 25% in the most aggressive companies when under extreme pressure ... [in the Korean War] the rate of fire [rose] to over 50%. However, no more than 20% of troops aimed their weapons.' Furthermore, 'Marshall's research (1947, pp. 48-9) indicated many troops will simply not use their weapons unless told to do so ....'
They need more peyote and/or ayahuasca!