Why did the radical environmental movement decline in the mid-2000’s in the U.S? Given the threat posed by global warming, why haven’t we seen the emergence of more intense contentious tactics from the movement? I develop a theory of why radical movements emerge, and why they collapse. I argue that pre-existing activist networks, along with the salience of an organizing threat like climate change, influence the emergence and tactics of radical movements. But repression changes a movement and has both a direct and indirect effect. Arrests and threats of repression can lead activists to exit the movement or change tactics (direct effect). As new activists from different networks enter the movement, they bring their own norms and preferred tactics (indirect effect). To test this theory, I take a mixed methods approach to the U.S. radical environmental movement and provide evidence from: 1) a new dataset of eco direct actions from 1995-2022; 2) novel survey data of the American public, younger Americans, and activists; and 3) semi-structured interviews with past and present activists and experts. I show the importance of pre-existing networks and subcultures as a pathway into radical activism, and how the response to repression shaped the subsequent trajectory of the movement.