United Nations chief Ban Ki-moon regularly spoke to international leaders and powerful diplomats. But in 2014, he faced a very different audience: a stadium filled with 30,000 anguished Rwandans who, two decades earlier, had seen more than 800,000 of their fellow citizens massacred in a hundred days of terror that have since been recognized as a genocide.

“We must not be left to utter the words ‘never again,’ again and again,” said Ban, as survivors screamed and sobbed.

The Rwandan massacre was the first time in history that an international tribunal had convicted anyone of genocide—a crime whose definition was only formalized in the wake of the Holocaust. Russia’s 2022 invasion of Ukraine has rekindled modern debates about the definition