The dehumanization of enemies in public discourse and within attitudes among soldiers is a common feature of many 19th and 20th century conflicts, enabling some of the great atrocities we've seen in the last 100 years. I am primarily familiar with the modern era, and the literature I know describes 20th century mass murder and dehumanization as a product of scientific, political and cultural thought of the last 2-300 years (I'm thinking mainly nationalism and racism here). But is there precedent for this in pre-modern military conflicts as well, especially before the Age of Discovery and military conflicts with visibly different peoples? Any clarification would be greatly appreciated.
"Dehumanizing" is a pretty loaded term, since it depends upon a very specific sense of being human. "Human" is, of course, a remarkably flexible category. Is a fetus human? A stem cell? Someone with a physical or neurological disability? Someone in a persistent vegetative state? Someone on death row?
These are all extremely difficult questions, and the way we answer them tends to touch on two linked issues: (1) Do we identify ourselves as sharing humanity with the fetus, criminal, etc.? and (2) Do we see that fetus, criminal, etc., as deserving certain human rights? As a US citizen, the way I answer the above questions directly impacts whether I think a fetus or a disabled person is entitled to the same life, liberty, and pursuit of happiness as me. That is, my idea of being human is not a strictly biological category. It's bound up with ideas about "We the People" and "certain inalienable rights" and "life, liberty, and property."
This idea that a shared sense of humanity also entitles people to share in common rights came together in the way that we think about it today during the Enlightenment (1700s). So from a somewhat pedantic perspective, we wouldn't expect dehumanization to be an issue prior to about 1700. Even though a premodern soldier might share biology with a person he was trying to kill, he wouldn't necessarily think that that shared biology justified the other person's right to keep on living. And given the sanitation, health care, and life expectancy of many premodern societies, it might have been tendentious for anyone to assert a fundamental "right to life."
But that answer is perhaps too pedantic, and maybe not quite satisfying. When we look at premodern sources, we do often find authors trying to diminish their enemies in some way as if to justify violence against them. Sometimes they even express this difference in terms of human nature. It's not necessarily dehumanization in a strict sense, but it's something darn close.
As an example from something I'm working on right now, Einhard's Life of Charlemagne, written ca. 830, recounts Charlemagne's reign of almost constant warfare. Einhard spent several chapters talking about the particularly violent Saxon Wars (772-804), and some of this was first-hand knowledge. He joined Charlemagne's court in the 790s during a period of bitter fighting, and there's no reason to assume that Einhard didn't himself go out on campaign. Of the Saxons, Einhard says:
Now the Saxons, just like nearly all the tribes living in Germany, are ferocious by nature and dedicated to a cult of demons, contrary to our religion. They judged it no dishonor to violate and transgress the laws of god and man. As such, they were able to disturb the peace daily since our borders and theirs are contiguous and flat nearly everywhere, except for a few places where great forests or mountain ranges divide the fields with a definite boundary. Murder and rape and arson continued without end on both sides. These things so aggravated the Franks that they judged it fitting not to continue in turn but rather to take up an open war against them.
So Einhard definitely sees the Saxons as being different by nature. In fact, he goes so far as to say that the Saxons are subject to neither human nor divine law! In his eyes, this ultimately necessitates conflict, but Einhard (perhaps unwittingly) also admits that the Franks are just as bad. The Franks go tit for tat with the Saxons in murder, rape, and arson, but then they unilaterally escalate to open war.
Is this dehumanization? It's pretty hard to say. Einhard was writing about thirty years after the fact, which is the space of an entire generation. (Early medieval life expectancy was about 35.) He's trying to justify why the Franks went to war, and there's no way of knowing whether this rhetoric was also used to encourage soldiers during the war itself. But for anyone who had participated in those wars, which certainly included some awful violence, this rhetoric perhaps helped assuage any lingering guilt. (PTSD is an entirely different question.) So it seems like Einhard's description of the Saxons may have served many of the same purposes that dehumanization achieves in the modern period, even if it wasn't dehumanization per se.