Day X

This story began with a mysterious gun in an airport bathroom and led to a German military officer facing trial on terrorism charges. It’s a story about national identity and reckoning, mirroring the story of Germany itself. And years later, it’s still unfolding — raising a question that democracies across the world are waking up to: What happens when the threat is coming from within? See all episodes of Day X at nytimes.com/dayx