“There was anger here, but also a lot of beauty,” Ms. Valade said. “So I continued.”

Not long afterward, following the terrorist attacks in Paris of 2015, France declared a state of emergency that gave the authorities the right to ban public demonstrations. With that, Ms. Valade had found her theme. Over the next two years, she shuttled around Europe, combing through web discussion groups for names of people who had participated in the Spanish equivalent of the Occupy movement, and tracking down a family of zealous environmentalists to a remote dwelling in southern France. Where possible, she embedded herself with her subjects, winning their trust, debating their ideas and ideals.