In parallel with the religious faith that sustain people like the Benedictines, there is a second, factor that has also served to motivate people: honour.
Most people are absolutely terrified of being wounded or dying. Yet for as long as religion has been breeding saints who overcome their frailties through faith, the military has also been able to create heros who do much the same thing out of a sense of honour . Indeed, if you look at the historical record, the ability to inculcate a sense of honour in a nation's soldiers has often been the strongest indicator of military success. In the age of black powder armies, for example, battle usually consisted of two armies walking very close to each other and blasting away until one side lost its nerve and ran away. The question is, why didn't the soldiers always immediately funk-out and run? The secret wasn't any fear of being caught and executed (18th century armies all suffered huge desertion rates---but that was usually between battles, not during them), but rather because of morale or esprit de corps. That is, the officers discovered that through a certain type of training the soldiers in their armies could so identify with the regiment that they would be more concerned about besmirching its honour than losing their life.
This rearrangement of priorities did not only apply to enlisted men. Officers were expected to expose themselves to as much danger as the men and routinely did; and just as routinely died. The concept of honour was a very important issue to them and they really did put “king and country” ahead of their own personal interests. Consider the dying of words of General Wolfe:
"See how they run." one of the officers exclaimed, as the French fled in confusion before the leveled bayonets.
"Who run?" demanded Wolfe, opening his eyes like a man aroused from sleep.
"The enemy, sir," was the reply; "they give way everywhere."
"Then," said the dying general, "tell Colonel River, to cut off their retreat from the bridge. Now, God be praised, I die contented," he murmured; and, turning on his side, he calmly breathed his last breath.
Modern warfare isn't as cut and dried as in the time of black powder muskets, but it still requires a level of sustained courage that goes beyond normal experience. It takes a lot of “guts” to ride around in a LAV, never knowing if the road beneath your feet will explode. Similarly, it is enormously trying to engage with a populace never knowing if someone in the crowd is wearing a vest made of TNT. In its own way, going out on patrol in Afganhistan is just as nerve-wracking as standing in a “thin red line” while waiting for the the officer to tell you to fire at the approaching French columns while the artillery and skirmishers kill men all around you. In both cases study after study has shown that the reason why men will endure this sort of suffering is not because of patriotism or some other reason, but instead one of “not letting the other guys down”, which, in essense, is what “esprite d'ecorps” or the “honour of the regiment” usually boils down to.
Modern armies have created a culture that goes to enormous lengths to create this sense of honour. In fact, it is a key element of the basic training experience---along with teaching the soldiers certain skills, how to take orders and raising their level of physical fitness. Going over obstacle courses, being yelled at by sargeants, bashing squares, and spit-and-polish rituals, are only of very minimal value in actual battle. Why they have been retained by all the militaries of the world is because they inculcate an emotional reprioritization where soldiers put the good of their regiment ahead of their own personal safety. And it is that reprioritization that allows officers to give suicidially dangerous orders to their men with the confident expectation that they will in fact be be carried out. This is the ultimate authority in military discipline, and discipline is the mother of victory. In effect, military power ultimately comes from how deeply our soldiers honour and love their regiment.
Just as the Benedictines were a social institution that developed faith and love of God as a mechanism to overcome people's inherent inability to live together communally, so the armed forces are able to use honour to similarly overcome the terror of battle.