LTC David Grossman wrote what we probably all know as 'the parable of the sheepdog' in his book On Combat (which is a fit companion for his work On Killing). The parable is featured in the movie American Sniper, but it isn't actually old enough for Chris Kyle (who was approximately my age) to have heard it as a child as depicted. However, there's a very similar saying usually cited as being from Heraclitus, who was a pre-Socratic philosopher.
Out of every one hundred men, ten shouldn't even be there, 80 are just targets, nine are the real fighters, and we are lucky to have them, for they make the battle. Ah, but the one, one is a warrior, and he will bring the others back.
So to return to the puzzle we finished the Laches with, what is the quality you're looking to teach, or train, or habituate -- and since it isn't, apparently, skill at arms or tactical proficiency, how do you get the benefits of those without hurting the real virtue you wanted?
When I was spending a lot of time with the Third Infantry Division's headquarters unit in Iraq, I learned from their division historian that they had been part of a pilot program when the American Expeditionary Force was being developed for World War I. Sharpshooters had been a regular feature of military units since at least the American Revolution, and in Europe since the Napoleonic Wars, but for the most part training in rifles didn't try to make each soldier a sharpshooter. It was thought that this was a quality similar to the one Heraclitus was describing: an in-born skill or capacity that couldn't be taught to most people. Instead, European armies tended to teach formation fire, clusters of fire, or by WWI the use of machine guns to mitigate the need for human accuracy with volume of fire.
The Third Infantry Division decided to train each soldier to shoot with significant accuracy. This worked very well, as it turns out: the German divisions they encountered outnumbered but were repulsed by them.
With nearly twice the personnel in the region than their enemies, the German forces could still counter the Allied forces in Reims, but French troops obliterated German forces in the area while taking little damage amongst their ranks. As this staged attack went on, German units were working their way up the Marne River, until they encountered the 3rd ID.A frenzied fight, known as the Second Battle of the Marne, ensued with 3rd ID holding their ground against an overwhelming German force. Despite throwing tremendous assets at the Allies in this region, the German Army’s only success was capturing the city of Mezy, which lies along the Marne River. However, by July 17, the Allied forces were able to seize back the city before the enemy could advance into Paris.... The 3rd ID proved to be a cornerstone of the defense of the Marne River and the entire region. The division’s valiant stand against a large German force marked a turning point in the war. This proved to be the last time the German Army was on the offensive during World War I. As a result of their heroics during this battle, 3rd ID troops came to be known as the “Rock of the Marne” and their motto “Nous Resterons Là” (We Shall Remain Here) were cemented.
So this is Aristotle's point about professional soldiers -- he meant mercenaries, as opposed to citizen soldiers -- being more successful because they understand more about what to do in the case of the battle. It does seem like there's a skill being trained here that is useful, which is why we still spend so much time and energy on professional military training. We have managed to overcome Aristotle's concern about professionals electing to abandon their posts because they understand that their position is hopeless by creating a class of citizen soldiers who are also professional soldiers. (The Spartans had done something like this too, even in Ancient Greece.) They will die for their country's honor and interests if they must, but they still receive the benefits of professional military education.
However, Heraclitus' point also stands this examination. Yes, the training helped many men in the 3rd Division. Yet the most famous soldier of that conflict was the 82nd Division's Alvin York -- a man whose skill at arms came from rural Tennessee, not the 3rd Division's pilot program. You can't make a man like Alvin York; nor was he trying to do so himself. He wanted to be a conscientious objector, not a fighting man. Somehow he proved to be the one Heraclitus was talking about in spite of undertaking no program to try to become so.
We have spent a lot of time trying to understand what qualities such people have, and whether we can predict who will turn out to have them. LTC Grossman's books are some of the best of these efforts. Yet it is still a mystery: and whether there is something here that can be taught or cannot be is still not fully understood.