There has already been such an agreement (The Hague Land Warfare Order, https://en.wikipedia.org/wiki/Hague_Conventions_of_1899_and_1907 https://en.wikipedia.or

There has already been such an agreement (The Hague Land Warfare Order, https://en.wikipedia.org/wiki/Hague_Conventions_of_1899_and_1907 https://en.wikipedia.org/wiki/Hague_Conventions_of_1899_and_...), but "cyber warfare" wasn't a thing then and thus not included.