A Russian soldier has just been sentenced to life in prison for committing war crimes in Ukraine. But what is a war crime? When one nation is fighting another, what are the rules, who can be found accountable for breaking them, and how can they be enforced? On today’s IDEAS, we examine the Geneva Conventions, the Nuremberg Protocol, and the other tools for governing the actions taken during warfare: how these tools work, what can be done to make them more effective, and the limits involved in implementing them.