I can only speak with my training in US history -- there is much more which could be said here from the perspective of European history. But seeing as it's been a few days and no answer has yet been written, I humbly propose the following narrative.

We can take Abraham Lincoln's General Orders No.100, or "Lincoln's Laws of War" aka the "Lieber Code", issued by Lincoln to U.S. armies in the field in April 1863, as one of the first codifications of legal limitations and guidelines for the prosecution of war. Among its 157 articles and provisions, Lincoln's Laws of War outline how prisoners of war should be humanely treated, guidelines for Union soldiers to avoid wanton slaughter, how to distinguish between noncombatants and civilians supporting the enemy war effort ("armed prowlers", spies, disguised partisans), and how noncombatants should be treated compared to military targets.

Some of the general principles outlined are the following, which I am pasting because they are just so interesting:

Art. 21-23: "The citizen or native of a hostile country is thus an enemy, as one of the constituents of the hostile state or nation, and as such is subjected to the hardships of the war....Nevertheless, as civilization has advanced during the last centuries, so has likewise steadily advanced, especially in war on land, the distinction between the private individual belonging to a hostile country and the hostile country itself, with its men in arms. The principle has been more and more acknowledged that the unarmed citizen is to be spared in person, property, and honor as much as the exigencies of war will admit."

"Private citizens are no longer murdered, enslaved, or carried off to distant parts, and the inoffensive individual is as little disturbed in his private relations as the commander of the hostile troops can afford to grant in the overruling demands of a vigorous war."

Art. 68: "Modern wars are not internecine wars, in which the killing of the enemy is the object. The destruction of the enemy in modern war, and, indeed, modern war itself, are means to obtain that object of the belligerent which lies beyond the war. Unnecessary or revengeful destruction of life is not lawful."

As an aside, it is perhaps ironic that from the U.S. Civil War, we see the articulation both of axiomatic laws of war and the nature of total war. Indeed, "military necessity" and the "demands of war" are baked into General Orders No. 100, and General Sherman in his March to the Sea campaign would articulate that war must be waged upon the morale of civilians as much as it should be waged on military targets.

But there are two important things to draw from Lincoln's 1863 Laws of War. Firstly, they would become the basis not only for the U.S. military's contemporary codes of conduct, but also in the articulation of laws of war across Europe as well. The Hague Conventions, convened in 1898, drew heavily from the U.S. Civil War code in setting out regulations for wartime behavior, including drawing distinctions between civilians and military targets.

Secondly, Lincoln (or Franz Lieber, the professor who prepared the initial drafts of the laws of war), did not poof these laws into existence. They clearly reference, in passive tense, the "acknowledgement" of general principles and the general advancement of (Western) civilization's sensibilities that leads the U.S. government to take the next step of codifying those sensibilities. These laws were products of the intellectual lineages of the time--namely, the Enlightenment.

If we look further back to the text of the Declaration of Independence, many of the complaints against King George refer to the unprofessional, wanton conduct of British troops. Jefferson specifically chose to include the complaints of British troops quartering in private dwellings, the British use of "mercenaries" who cannot be trusted to act with civility, the summary impressment of American colonists on naval vessels; all of which reflect a *sensibility* that the improper conduct of the British military reduced the legitimacy of the British government, and therefore an independent Continental Congress could claim a higher standard of virtue. One's claim to the mandate of the people, in part, derived from an equitable relationship of the military to a civilian populace, not just power alone.

This is where limitations in my training--which is woefully inadequate in Western intellectual tradition--preclude me from drawing general conclusions. But I think it safe to assume that the 18th and early 19th centuries saw the intellectual development of moral principles that apply even in wartime.

This would track with the development of a modern, professional army which expand on the idea of proper conduct and professionalism in the labor of soldiering. As recounted in William Skelton's work An American Profession of Arms, the history of the curriculum of the U.S. military academy at West Point shows as much. The USMA, founded in 1802 as an engineering school but expanded into a general training ground for officers under the tenure of Superintendent Sylvanus Thayer (1817-on), began to include works by the jurists Emer Vattel and James Kent in their standard curricula, which covered international law and ethics. The officer corps in the US was very small and who can say the impact that such limited training would have had in reality, but the linkage between professional soldiering and a legally-aware mind was clearly there.

To end this off, a recurring paradox in the idea that there ought to be limitations and distinctions between legitimate targets in war has been the existence of an exit clause -- military necessity. General Orders No. 100 suggest that military commanders should warn towns prior to a bombardment, but also indicate that surprise might be a military necessity. Or, even further, the exclusion of entire groups to having the standing to lay claim to their protections under laws of war. General Orders No. 100 did not stop the slaughter of American Indians, both noncombatants and combatants, on the American frontier. Nor did it seem to have much purchase with regard to the conduct of American troops in the Philippines during the Spanish-American-Philippine War of 1898. General Jacob Smith would order his troops to turn the island of Samar into a "howling wilderness", who subsequently engaged in wholesale slaughter of civilians, essentially as a revenge for the grinding insurgency. The War Department would eventually court-martial Smith for the minor crime of unprofessional conduct, who remained incredibly popular despite ordering mass murder.

The "sensibility" that civilians ought to be spared the horrors of war where possible was the product of a long Enlightenment tradition, codified by the U.S. Civil War era- Laws of War. It tracks alongside the notion that there is such a thing as a professional military with officers must be trained to have good conduct, and crucially matches the creation of ever more violent and destructive weapons and tactics of war.

​

Sources:

General Orders No. 100, https://avalon.law.yale.edu/19th_century/lieber.asp

William Skelton, An American Profession of Arms.

Paul Kramer, Blood of Empire.

Hague Convention of 1899 Laws of War, https://avalon.law.yale.edu/19th_century/hague02.asp