Collaboration with the enemy is a common and often painful part of armed conflict. It is also an issue in which I have both a professional and personal interest. The war in Ukraine is, in many ways, a transparent conflict, with cellphone images, drone cameras and satellite imagery feeding a flow of data to social media platforms and news outlets. But in Ukraine’s occupied territories, there are actions and decisions that many people – ordinary residents and officials alike – will want to remain hidden, not just for now but for years to come. I am a scholar of international...