The Importance of Nerves of Steel in Military Training

The training of the Spartans was so much more difficult than the the training of modern soldiers. would kind of push the enemy as one with our shields and then we would open up, stab and then close our shields back up. If the god of your right gets scared and runs suddenly your flank is exposed and you're going to dieso it was right super incredibly important that everybody had nerves of steel how do you get people to be like that?