I think people are seriously underestimating either sides ability to gain knowledge and adapt.
It is amazing how fast you learn about someone when they are actively trying to kill you.
Not me- but I also recognize their limitations.
1) Looking at the arcane invaders: assuming a mixed race force means fewer individuals who look human enough to infiltrate. Magic alleviates this somewhat, but not completely. Other means like screwing have limitations of range and their own peculiarities.
On the tech side, we have ridiculous satellite imagery, but it doesn't get you audio. By definition, there would be no transmissions epwe could monitor. Special forces can do some infiltration, but will have limitations of their own.
2) There is the language barrier: Common probably isn't English...or Chinese or any other language of our world, so your cryptographers would have to go to work. This isn't easy for them, either- remember how much trouble the Germans had breaking transmissions in Navajo and other Native American tongues? So an infiltrating agent- assuming he could pass off his inability to communicate as a personal deficiency- would probably have only the most basic understanding of operations. He could relate all of what he saw, but none of what he heard. Bugs have the same limitation.
And, of course, the same problem exists for the invaders, only amplified by the fact that so little of what we do resembles what they would. A medieval/fantasy war camp operates very differently than a modern base...or even a mobile camp. An infiltrating invader would not see field forges or men actually building weapons. He could find the machinists, but would have difficulty distinguishing between parts destined for attack choppers and those meant for jeeps...or which machines are the most crucial for making those parts.
3) Then there is the nature of the knowledge you need to acquire.
Both tech and magic have analogous limitations: any truly dangerous knowledge takes a lot of time to acquire and understand. To fight magic with magic requires study of the language, rituals and rules of magic; to fight tech with tech requires actually understanding the tech.
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