Magic or Not To Magic.
As I see it Elves are very quiet and composed (generally not considering Drow & Wild Elves). I find it hard that Humans would be the ones defending. I mean an Elf and a Forest...AND magic...what else do you want.
Humans without magic are very strong. 200 commoners have two feats at first level. It could be that they are all trained in using light armor and shortswords (Pitchforks, sickles, Scythes, whatever you prefer). Human numbers grow like lice on an orc. There is virtually no end to their ability to "make babies". Humans also are more versatile than elves; therefore, the idea that they use more tech -- alchemy if you really want to maintain Middle Age feel -- is not a bad idea.
Elves on the other hand are slow reproducing, finite number of spell weilding folk. Flying mages with fireballs are conceivable, but what happens when you cast your flight, and 3 (more likely 2) fireballs. Now you've got a floating elf in the line of sight for a mile. Elves having their forest burned down in highly unlikely. Elves have magic to put out flames...not to mention the treants, pixies, dryads and other forest sentient who live in the forest as well.
Overall, I think a Cold War atmosphere would fall over the campaign. General dislike by either side. Although...there are still gnomes, dwarves, and (ack! dare I say it) halflings to consider. Gnomes aiding the techie Humans would be a scary sight to see. And if the dwarves were on the Human's side, now you've got enemies with spell resistance and built in blacksmiths. It would get to be a grim reality for the elves. But who is to say it might not be the other way around.
This is not even including divine magic, which they might not use; after all, if a god feels like intervening who is gonna stop him.