This Element provides the first large-scale inquiry into the 'Reopen' protest movement against COVID-19 public health shutdowns. We synthesize digital ethnography inside the movement with text analyses of an original data set spanning more than 1.8 million Facebook comments and posts from over 224,000 online activists. We characterize the movement's origin, growth, and evolution as it interacted with public policies and offline protests. We explain individual- and group-level dynamics of radicalization over time, across topics, and, paradoxically, in response to content moderation. We extend existing theories of contentious politics to suggest that movements that fail to maintain their connection to offline organizations are especially prone to mutability, radicalization, and exhaustion. Together, our findings offer a powerful theoretical framework for understanding social movements in the digital age, while updating and extending classical social movement theory.