How can rules of war be kept sacrosanct?

Question

After a great and terrible conflict on Earth, known as World War II to the planet's inhabitants, the various nations that survived the war put in place a set of guidelines which specified certain actions and weapons to be unacceptable in the course of a war, such as forbidding the use of poisonous gasses, or the intentional slaughter of innocent civilians. These guidelines became known as the Geneva Conventions, and they were, for the most part, upheld by the nations of the world. Primarily this was due to the so-called "Cold War" between rival superpowers bearing nuclear weapons, who feared each others' ideologies as much as a nuclear conflagration.

For a period of time after one of the global superpowers lost the "Cold War," it appeared that the remaining superpower would uphold the Geneva Conventions, and act as a pseudo-police unit for nations that did not. However, following a devastating terrorist attack on the superpower's largest city, it undertook a series of actions that directly contradicted the Geneva Conventions, such as condoning the use of torture. Other nations followed suit, and after a period of time, the Geneva Conventions were seen as a lofty goal at best, and an encumbrance at worst.

Therefore, I ask: what conditions could ensure that the rules of war be followed in a society of states where there is no undisputed hegemon? I exclude the possibility of hegemony as it is 1: unlikely in our culture for the foreseeable future; 2: not guaranteed to be gentle or just, considering how many people would have to be put down for opposing a single world government; and 3: there is little assurance that a traumatic event such as a massive famine or terrorist attack would not result in societal upheaval that is violently suppressed.

EDIT: I am only using Earth and human history as an example of how global rules can be imposed and later broken. The answer need not necessarily reflect the history of humans on Earth.

Answer 1

They cannot, unless you redefine war to not include total war.

It is generally accepted that war should be your absolute last resort, when all rule based solutions fail. Total war is the absence of a solution that includes rules.

Even when we talk about rules of war, it is recognized that they are not sacrosanct. The Geneva convention gets broken more often than we'd like to admit. When someone does break those rules of war, they are put on trial for war crimes... but that only starts after the war has been fought and won. The rules were still broken and there was nothing anyone could have done about it. Worse, if a side wins, they typically get away with war crimes that they would not get away with if they lost.

A famous example is the Zulu tribe of South Africa. The nearby tribes had developed a highly ritualized form of war with "sacrosanct" rules. When the Zulu came, they ignored those rules and brutally massacred many tribes' warriors before the other tribes realized the rules weren't applying anymore.

The only option left is to redefine war. Define war to have sacrosanct rules, and leave that-which-shall-not-be-named to describe what happens when those sacrosanct rules are broken.

Or, I suppose you could only make rules that are hard to break. Most warriors do not break the laws of physics, so you could start with those.

Answer 2

How about this?

• Many medium-sized states without significant ideological rifts. Alliances become interchangeable, people are not concerned if their province changes from one ruler to the other. Perhaps perhaps it is normal that territory is exchanged at the marriage of kings/merger of megacorps/whatever.
• A general understanding that war is fought for a few border provinces, not the destruction of one or the other government or nation.
• An unwillingness to raise nationalistic fervor in the masses, perhaps because that might cause them to question the status quo.

On an interstellar basis, something like this happens in the CoDominion series by Pournelle, the prequel to the more famous novel The Mote in God's Eye by Niven and Pournelle. Pournelle's setting was tailor-made for stories of "civilized" warfare, and to show what happens if this "civilizing influence" breaks down.