> The fact that a person acted pursuant to order of his Government or of a superior does not relieve him from responsibility under international law, provided a moral choice was in fact possible to him.
Of course the bigger players all aren't members (US, Russia, China, India). But WWII Germany wasn't part of anything either and we still had the Nuremberg Trials, so it's very much "international law doesn't apply until suddenly it retroactively does".
It's noteworthy though that the ICC's stance on the Superior's Orders defense is not completely clear for the less severe cases.
[1] https://en.wikipedia.org/wiki/International_Criminal_Court
You are drawing an arbitrary line in declaring "which laws matter." In reality the lives of the people are governed by which laws are most effectively enforced at a local level.