Searching for a Winner: Review: Axioms of Warfare: The Objective; The Offensive; Mass
In the review of Mass an excerpt from Pressfield’s Gates of Fire is included.  It stresses the source of fear and the reason the shield is the preeminent weapon among the arms of the Greek hoplite.
Pressfield, Steven.  Gates of Fire.  (New York: Doubleday, 1998), 34-36:
Dienekes began the sequence of query and response which comprises the Lakedaemonian syllabus of instruction.
“Answer this, Alexandros.  When our countrymen triumph in battle, what is it that defeats the foe?”
The boy responded in the terse Spartan style, “Our steel and our skill.”
“These, yes,” Dienekes corrected him gently, “but something more.  It is that.”  His gesture led up the slope to the image of Phobos [ FÒboj, “fear” ].
Fear.
Their own fear defeats our enemies.
“Now answer.  What is the source of fear?”
When Alexandros’ reply faltered, Dienekes reached with his hand and touched his own chest and shoulder.
“Fear arises from this: the flesh.  This,” he declared, “is the factory of fear.”
Alexandros listened with the grim concentration of a boy who knows his whole life will be war; that the laws of Lykurgus forbid him and every other Spartan to know or pursue a trade other than war; that his term of obligation extends from age twenty to age sixty, and that no force under heaven will excuse him from soon, very soon, assuming his place in line of battle and clashing shield-to-shield, helmet-to-helmet with the enemy.
“Never forget, Alexandros, that this flesh, this body, does not belong to us.  Thank God it doesn’t.  If I thought this stuff was mine. I could not advance a pace into the face of the enemy.  But it is not ours, my friend.  It belongs to the gods and to our children, our fathers and mothers and those of Lakedaemon a hundred, a thousand years yet unborn.
“Have your instructors taught you why the Spartans excuse without penalty the warrior who loses his helmet or breastplate in battle, but punish with loss of all citizenship rights the man who discards his shield?”
They had, Alexandros replied.
“Because a warrior carries helmet and breastplate for his own protection, but his shield for the safety of the whole line.”