system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is war not a war crime? Where do you draw the line? Torture? Mustard gas? Why not killing? Nukes?
gpt
In theory, war itself is a diplomatic state between two nations, where in those two nations will use their respective militaries to sort out what ever disagreement brought on the state of war.

War crimes are, in theory, defined by a breach of the Geneva convention, which are the rules by which nations are required to make war (  pretty much, leave the civilians alone, and don't nuke, gas, or use any weapon that will cause unnecessary suffering.), in theory.

In practice, the worst offenders tend to win the wars they're involved with, rendering the Geneva convention useless as a tool to restrain the excesses of powerful nations at war, And reducing it to a tool to further punish people who have lost a war.