Well, I'm definitely not qualified to answer this definitively, but I can give you a few things to consider. I'm a philosopher who has done a lot of work in history, and with a specialized interest in ethics, so this is going to be primarily from the perspective of philosophical works that have discussed these ideas (of course, whether or not that 'factors' into the decisions made by military and political leaders is a different story!)

In Plato's Apology he recounts Socrates defense against charges of impiety and corrupting the youth, and one of the defenses Socrates offers is that he refused to kill Leon of Salamis as punishment after the Peloponnesian War even though he was ordered to by The Thirty (the tyrants who temporarily ruled Athens.) (Apology, 32d - 33) So even in very ancient warfare there were ideas about right conduct; if you read The Iliad, it is almost solely about right or noble conduct in war, despite being pretty unbelievably brutal. So the idea that you could behave badly in times of war is not at all new.

Similarly, Sun Tzu's Art of War has long passages about the necessity for winning battles decisively, so that the war can be quick and result in less harm to the farmer class that is so threatened by warfare.

But if by 'ethics' in war you mean something closer to what we recognize in the contemporary period, in which an idea of individual human rights is central, then I can recommend Michael Walzer's book Just and Unjust Wars: A Moral Argument with Historical Illustrations. It does a nice job of highlighting some of the relevant ideas such as the philosophical difference between ius ad bellum (justice in going to war) and ius in bello (just conduct during war).

Walzer is a contemporary philosopher, and he is not coming up with these ideas on his own - Augustine of Hippo (around 400 AD) in City of God wrote about just war doctrine and heavily influenced Thomas Aquinas (around 1250 AD) in his work on just war and the doctrine of double effect (this is the doctrine that discusses the ethical implications of performing an action for a specific purpose (i.e. destroying an important bridge that the enemy is using to re-supply) even though you know that another effect of this action will be to cause harm to innocent civilians.

So I guess a takeaway is that at least from a philosophical perspective, the idea of ethics has factored into our ideas about warfare for a very long time. What has changed (and hopefully is continuing to change!) is what is considered ethical and what is not.