At noon on August 9, 2014 when Michael Brown was killed on Canfield Drive in Ferguson, there was little protest. But by 9 pm, dozens were nonviolently defying police armed with military style weapons, armored vehicles, helicopters, and snarling dogs. The structural situation alone cannot account for the emergence of insurgency in Ferguson. To explain mobilization, I advance a theory of Contested Legitimacy. The stakes of each action by insurgents, authorities, and third parties for mobilization concern regulatory repression. Actions that undercut the validity of repression encourage mobilization. Video, photo, and textual data make it possible to unpack the complex interactive process of mobilization. Given longstanding grievances concerning racist policing in Ferguson, reclaiming the site where Michael Brown was killed on Canfield Drive as a memorial provided means to challenge unjust police authority. When police responded as accustomed- disproportionately, callous, and indiscriminate - their actions galvanized local Black support for activists.