Again?

Another one for the, "I didn't think I needed to explain this, but here we go..." file.

"Nothing is a war crime the first time."

It's a pithy statement describing how the rules of war are changed as wars are fought and everyone afterwards decides, "nope we're not doing that again."

All laws are such, actually.

You don't need to pass a law saying something is wrong until someone goes off and does something that you had not considered anyone would do.

Looking ahead and deciding, "nope, that thing nobody has ever done before, is wrong," is built on a foundation of hindsight from being blindsided.  You can talk yourself into passively submitting to evil because you're not willing to do what needs done to stop it because what you will need to do is "wrong".

Canada has made noteworthy contributions to the Geneva Checklist.

There's a few things on that checklist that were SOP until the technology of the time changed the scale of the action and made it horrifying.

Two atomic bombs that did less damage each than firebombing Dresden or Tokyo isn't a war crime.

Realizing that two nations who're not fond of each other possess enough nukes to make the entire world uninhabitable and will use them against the other if even one is uncorked is a difference in scale that makes using them evil.

It's not the actual use of the nuke, and it's effects on the target, that are the problem.

It's the use one, start MAD that's the problem.

An important hint is the revolutionary system Rome used on the people in places it conquered involving letting them live.  Ask yourself what happened to such folks BEFORE this revolutionary system was implemented.

Genocide, though the word had not yet been coined.  The lucky ones were enslaved, but their culture was wiped clean.