To a certain degree, there have always more or less been "rules" or anyway customs governing warfare. The custom of ransom gives a losing war chief an incentive to throw in the towel before his forces are completely routed, for instance, saving the lives of some soldiers and ending a conflict when one side seems to have prevailed. Obviously the force and effect of such customs fluctuates a lot. The Catholic Church in medieval Europe systematically attempted to regulate warfare by all sorts of expedients--attempting to ban fighting on holy days, including trying to make Sunday and as many weekdays as they could try to get away with "holy," etc, offering to mediate, and so on. I would expect that study of the history of Islam offers comparable examples of mitigating customs between Islamic contenders; in fact I know of some examples of this kind of thing. (And even between Crusader and Islamic defenders, such customs also evolved or were invoked, having already been developed and observed).
Since the Geneva Conventions also are observed only partially, I'd characterize them as the modern version of this perennial tendency to civilize warfare. Their effectiveness varies depending on how existential a fight seems to be to one side or the other, the nature of the coalitions on each side, whether they are fighting members of an "in" group or an "out" group, whether Great Powers that one side or the other depends on have a stake in the conflict (which can work either way--either a Power gives a smaller one diplomatic cover for extreme measures because the Great Power thinks the stakes are high, or it influences its proxy to moderation because the stakes of a greater conflict are higher) and so forth.
I'd think that for warfare to go on but under strict rules would require a super-state or de facto coalition of them that recognize the importance of limiting the destructiveness of conflict, but also positively value a certain amount of bloodletting. (Orwell's scenario in 1984, for instance.

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A dominant system of powers ruled by people with a genuine abhorrence of war or anyway a healthy fear that conflicts however small and limited might spin out of control with disastrous results would look to suppress conflict completely.
Of course a possible situation is a power that hates to see fighting in its own sphere but feels a need to maintain forces for a knock-out fight with someone outside its sphere; one measure the Catholic Church took in the early Middle Ages to try and drain intramural fighting in Europe was to send surplus nobles off on the Crusades; all-out warfare against Muslims was seen as a good thing in itself. (Martin Luther had some interest in a joint Protestant-Catholic Crusade, so I've read somewhere in my Crusades class texts IIRC). This in turn can lead to a certain tolerance or even advocacy for violence as an option in one's own sphere, the better to maintain a body of people skilled and steeled in combat to periodically drain by turning them against the common foe.
For the Geneva Accords to be fully enforced around the world, I'd think we'd have to have a world government that was acceptable enough to all peoples that war would be effectively abolished completely.
Falling short of that, we would most likely be looking at two or more superhegemonies in existential but possibly "cold" war with each other that enforce no-war order within their coalitions, and more or less agree on following the Conventions for mutual interests in their proxy wars. Which I suppose does describe the OTL Cold War, but note that proxy forces (which OTL often were not in fact under the effective control of their supposed patron) often broke the rules at their whim.
Which in turn leads to practices like "extraordinary rendition." The agencies of an enlightened great power nation won't practice torture, oh no! That's against a bunch of laws and court decisions. But this guy they caught is bound gagged and blindfolded on an airplane headed to the ally Ruritania, which has, shall we say, laxer rules...
So in response to your challenge, I'd say it has been met again and again, in OTL history, depending on how strictly and sweepingly you specify the regime should reach, and how many of the breaches of the modern regime you compare it to you are willing to overlook, or are compelled to overlook because the facts have been successfully suppressed.