Celebrim
Legend
So what I largely take away from this, is to just do as I see fit in my world.
That is and always has been the prerogative of the Game Master.
Since it's not really going to be set to any time period it doesn't really matter how historically accurate I am trying to be.
Any historical rigor you apply to the setting is always only to make yourself happy. My advice on this is usually to use history as a resource to increase the richness and texture of the setting, and not as a straight jacket that limits your creativity. For a fantasy game, history ought to be inspirational and not proscription.
That said, part of the reason the Greyhawk map is such a better map than many of the fantasy D&D maps that came after it, was Gygax was through his background in historical wargaming much better prepared to create a pastiche of historical and Medieval culture in particular in all of its richness than many later authors were.
I guess I just think of WW1 and WW2 and how much they effected entire countries and I want to carry that over into my world.
It's my opinion that most people are incapable of truly imagining a culture more than one or two centuries removed from their own, and so whenever they create 'historical fiction' they almost inevitably imagine that the historical world they are imagining was a lot like what they think the world was like 100 or 200 years ago. Thus, Greyhawk was at the same time both a Medieval culture (based on Gygax's writing) and at same time the Old West of 19th Century America, which was the romantic past as Gygax understood it. Gygax however was at least conscious of this, where as many other D&D writers (and many writers in other mediums as well) haven't had the education to actually be conscious of their anachronisms.
Write what you know, but I would encourage you also to read widely in history - it helps you be a better GM by helping you present a more believable, more alive, and more interesting world.