I think @old-and-new-friends is right that the Avatar world would have of its own conventions and understandings of war crimes, and more broadly what we would call ‘just war’. I think their contents probably would vary, perhaps quite dramatically, from what we think about. TL;DR on the ramble below - I think much stricter controls on the reasons for going to war and who gets to declare war, but less protections on civilians than we might expect.
So in the Chinese and Japanese tradition of military ethics (from what I understand - I am not a scholar of this field!) war is seen as a way to achieve harmony - Chinese military text the Wuzi, written in 4th century BC, says that war “suppresses the violently perverse and rescues the people from chaos.” It’s a similar story in Japan; war is framed as the suppression of criminality that breaches the peace established and maintained by the court. It was very much seen as the last resort after more peaceful offices and influence have failed, and is something to be deployed against ‘barbarians.’
I think this jives pretty well with Avatar. For example, the Harmony Restoration Project, while not outwardly violent, is a major move justified as something to restore harmony; and that is the Avatar’s role more broadly. I also suspect that countries put a lot of effort into finding ways to frame any military expeditions under this rubric in order to avoid the Avatar’s wrath.
A quick side-note on this, also, is that there are very few actors in Avatar, with only four nations which - with the exception of the Earth Kingdom - are relatively unified.* It’s certainly in their interests to have a system of military ethics which frames war as being something only central authority can declare, and which exists to quell criminal elements/restore harmony; rather than a regime-change kind of situation (although, obviously, Sozin flips the switch on that and we know from the Kyoshi novels that the Fire Nation military had contingency plans for large-scale operations against the Earth Kingdom hundreds of years before the war - so this is by no means hard-and-fast).
Quite interestingly, despite the strong emphasis placed on the right/moral conditions for going to war (what, in the western canon, is called jus ad bellum) both China and Japan had less to say about non-combatant protection. I draw particularly on Japan here, where it’s noted that civilians were accorded no particular protections because what was ethical was the overarching end-state of the violence - some brutality against civilians was not considered as incompatible with the broader restoration of the peace objective.
China had slightly stronger protections in this space; distinction between combatants and civilians was a necessary part of fighting a righteous war; but that was undercut by the identification of the targets of war as being those outside of normal society/protection - and hence not really non-combatants. This seems broadly to tally of what we see in wars in Avatar. The Fire Nation’s various activities are framed as wrong, but aside from the Air Nomad genocide, I don’t recall their imprisonment of benders etc being described as especially out of the ordinary. Kuvira’s troops have little regard for civilian protection but place a lot of emphasis on the greater end of unity.
I think another piece to suggest that civilian protection probably isn’t a big priority in the Avatar universe is the lack of organised and influential religion. We know that there are various groups of sages, but they compete both internationally and internally - especially in the the Earth Kingdom, see Jianzhu’s whole thing about Earth Sages. Historically, in Europe, the influence of organised religion through the Peace of God movement (among other bits and pieces) played a key role in civilian protection. The absence of such authority in Japan in particular - religious groups being fragmented and competing for secular power, much like the sages in Avatar - was a big reason why civilian protection wasn’t more of a thing there.
What does this unstructured ramble mean? I think it suggests that the norms of military ethics in the Avatar universe probably were heavy on why wars start and who declares war, but much lighter; if not entirely non-existent; regarding the protection of non-combatants. How that might change in the aftermath of the Hundred Year War is a post for another time (and requires more thinking!). At a guess, I’d say that Aang would insist on much stronger non-combatant protection, and possibly outlaw wars of aggression altogether as a crime against peace; certainly, the charges levied against Kuvira during her tribunal in Ruins of the Empire suggest something like this. But I’m not sure how extensive or codified such things would be, given the lack of infrastructure both physical and normative to support it.
My source on all this is The Ethics of War in Asian Civilizations: A Comparative Review, 2005, edited by Torkel Brekke, and also dipping into some generic background stuff on Thomas Aquinas and Saint Augustine.
* In actual fact, it’s probably even lower than that. The Southern Water Tribe doesn’t seem to be a major military power at any point in the canon, and the Northern Tribe appears highly isolationist. And the Airbenders, while certainly interested in peace and harmony, aren’t really geopolitical actors. Which really leaves the Fire Nation and the Earth Kingdom as the only ones who these sort of things would directly and routinely apply to.
