Hey /r/AskHistorians. I apologise if this is covered in the FAQ, I spent a while searching and came up with a few similar but not as specific questions.
Many modern media pieces depict any pre-WWII army as a collection of regular soldiers led by a noble, royal, or just better figure than the soldiers themselves. The Sharpe series for instance begins by showing how difficult it is for a common soldier to command with the same authority as a gentleman and this theme continues throughout every movie/book. The quote in the title was shown on a Total War loading screen, I have been unable to verify it specifically but Onosander was a Greek philosopher who wrote about the duties of generals and such.
First, did this superstition exist as the media portrays?
If so, where, when and how did it arise? How did it then fall and when exactly did it fall? I feel like it was some time between WWI and WWII, so I'm looking for an answer that can pinpoint an event rather than a time period.
How far did the belief that the aristocracy was "better stock" than the common man extend beyond the military and how did this affect human development? (This one is very broad and open ended and possibly against the rules, so please feel free to ignore it.)
This isn't directly answering your question, but it regards to the quote by Onasander itself. It's kind of important to take that quote in context.
It's from a larger and well known work called Strategikos (The General). You can read a translation here.
In that chapter, he's outlining the qualities of a good general. Here's a few relevant quotes:
The general should be a man of good reputation, because the majority of men, when placed under the command of unknown generals, feel uneasy. For no one voluntarily submits to a leader or an officer who is an inferior man to himself.
I think your quote specifically is a weirdly translated version of that last part:
For no one voluntarily submits to a leader or an officer who is an inferior man to himself.
That doesn't necessarily mean by birth. There's a good chunk below which can expand upon it:
An illustrious family name we should welcome, if it be present, but if lacking it should not be demanded, nor should we judge men worthy or unworthy of commands simply by this criterion; but just as we test the pedigrees of animals in the light of the things they actually do, so we should view the pedigrees of men also. For it is dangerous to consider what fine thing a general's ancestors have done, rather than what the generals now chosen will do, as if those long dead could still protect us, and as if they would maintain us in our former possessions. As a matter of fact, is it not sheer stupidity to honour soldiers for valour, not those of famous families but those who have done some noble deed themselves, but on the other hand to select generals, even if they are incompetent, on account of their ancestors and not on account of their own worth, even if their families are unknown? Of course, if a general has birth in addition to these other qualities, he is fortunate, but even if he has a famous name without the other qualities, he is useless. It might perhaps be expected that those men who cannot take pride in their ancestors would become even better generals; for men who glory in their forefathers, even if they are themselves failures, believing that the fame of their family is theirs forever are often too careless as administrators, whereas those who have no ancestral renown to begin with, desiring to make up for the obscurity of their lineage by their own zeal, are more eager to take part in dangerous enterprises. Just as the poor man, eager to supply what fortune omitted, will endure more than the rich man in getting a start to make his fortune, so the man who can avail himself of no inheritance of ancestral glory determines to make his own the virtue which he himself acquires.
Particularly good bit in there:
Of course, if a general has birth in addition to these other qualities, he is fortunate, but even if he has a famous name without the other qualities, he is useless.
So you can see, Onasander thought a general being well known/famous was good, but thought it would be a terrible mistake to pick a general on those merits alone. If you check out the rest of the work he lists more qualities to look for in a general and why they are important, but those seemed to be the most relevant to the nobility issue.
I feel like I broke the title somehow. Sorry about that, please fix it if you're able to.