[int_19h]

The notion that there are some "norms of civilized behavior" that go beyond states and their written laws is very old, dating back centuries and even millenia:

https://en.wikipedia.org/wiki/Jus_gentium

https://en.wikipedia.org/wiki/Hostis_humani_generis

It works in the same manner as unwritten common law does - set by custom and precedent.