Upvote:0
I'd put forward there was an inflection point that occurred at some point during the 19th century. It's probably not a single event, and one could probably go on at length as to the precise reasons.
You can see clearcut evidence of this on the eve of the Boxer War. In response to the atrocities in China, the German Emperor infamously urged his troops to 'behave like Huns', triggering outrage across Europe (p.873).
As little as a century or two earlier, it would have been so matter of factly - indeed expected - that troops would do so that it wouldn't have registered as something worth instructing.
Upvote:4
First, let's clarify; declaring something illegal does not prevent people from doing that thing. Burglary is illegal, but it happens. Sexual harassment is illegal, but it happens. So all of the examples listed are... not really relevant to understanding or answering the question. The examples lead more to confusion than resolution.
Second you are citing examples from "ancient Western civilizations" (unspecified, I'm going to assume Rome), then from Bismarck - that is a gap of nearly 2000 years. The Romans were rather enthusiastic about genocide Carthago delenda est! During the anarchy, there are examples of people killing hostages and of people behaving with honor. During the Hundred Years' War, war on civilians was permitted; perhaps because the civilians were serfs, and therefore not entirely people. Richard the Lionheart was ransomed and treated with dignity, as were most nobles. During the Napoleonic era, the French pursued scientific mass murder of civilians. If you want an answer to this question, you're going to have to narrow down the scope to what impact international law had on a specific war. If you ask the same question about any legal principle, the answer is book length. "What is the history of rules against burglary from Rome to modern intellectual property?"
With that as prelude, I think it is fairly easy to identify some inflection points that are critical to the distinction between civilians and combatants:
Pope crowns Charlemagne - establishing a basis for law that is distinct from the person of the autocrat. (Arguably, the Roman tablets did this first, but I'm not sure they were ever more than a symbol, and any discussion would have to develop Augustus and Domitian.)
Grotius, drew the distinction between law an natural law. (Again, one could argue the ius gentium but I pick Grotius because he articulates that international law is not divine in origin (and therefore applies even to those that local gods do not love). One could argue the ius civile is similar, but I think Roman law is tied to Roman gods, and the Romans are not known for their fundamental respect of the civil rights of non-Roman citizens.)
Some sources privilege the Roman Catholic Church's attempts to impose peace on various conflicts; I believe these are distinct.
Another inflection point is the creation of war crimes and crimes against humanity. A third inflection point is the international prohibitions against Letters of Marque and perhaps a discussion of the Condottieri. All of these influenced international law on conflict and the treatment of civilians.
In every war, there are some who want peace; some who wish to mitigate the suffering. Advocates of peace imposed restrictions on the conduct of war (check for the rules against crossbows, or fighting on holy days, or the requirement for a just war, etc.). Given the breadth of your question (two millennia over the entire surface of the globe) that may be the only honest answer to the question.