Armies are brutal. Conquering armies rape, destroy, and annihilate. Defending armies do the same. When a battle is fought, men lose their minds and become drunk on slaughter, letting it spill over every one and every thing. They treat friend and foe alike, killing everyone indiscriminately as long as they don't wear the same colors or a uniform at all. The Ancient and Classical worlds were no better than the Middle Ages, the Renaissance, or even the Enlightenment. Soldiering changed little; the chaos caused by war allows those who merely dream about wanton murder and theft the freedom to let loose. The overwhelming sensations of being near death, of winning a hard-fought victory, of murdering at every hand, lead otherwise sensible men to become monsters as well.
So what's the question? The question is: how would this change when your foe wasn't a person like you, but rather a wholly different creature?
The example I'm looking at here (because it may be relevant to my game soon) is the elvish army moving on Arovrienna and the ring of goblin encampments that have sprung up around it. If the goblins win, there's no question they'll rape and kill ilmai and aloisia landholders alike. These elves are foes, they look like a different race of people, and they provide sport for goblin egos (goblins in the 10th Age are one of the flawed "mud peoples" made by the Felnumen, or lesser gods, and their particular mental block is that they have no humility).
But what if the elves win? Will there still be cruel and vicious elf knights who may exploit the madness of the battle and ensuing war to further their own grotesque desires? Jocelyn, to whom I first pitched this idea, said to me: "Yes." My original thought was no. I'd like to examine both answers.
NO. The distinction between friend (insider) and foe (outsider) is too great. Fighting in a battle against an enemy race (or any other race, one that is not necessarily known as a foe) allows for immediate and complete identification of those who belong and those who do not. Therefore, there can be no confused emotions spilling over onto members of your own race. This is something like National Pride, but taken to an absurd level. Elves protecting other elves from goblins would certainly feel as though they are doing something noble and thus wouldn't stoop to the level of sullying their victory.
YES. So what if you can identify the outsiders? There are people who simply use the swirling maelstrom of the aftermath to do whatever they please. This isn't going to stop because they were fighting goblins instead of other elves. While an elvish army may be among the most controlled and disciplined, you're still going to get those who feel their emotions wash over them (particularly amongst the soulful and lusty elves) and allow their reason and judgement to be overriden by the madness of war. Just because identification of the outsider is simple doesn't mean, in this case, it makes any difference.
I so want to go with no. But I fear the answer is yes.