Between 2011 and 2016, the Islamic State successfully convinced tens of thousands of individuals around the world to join its ranks. Many attribute this surge in foreign recruits to the group’s sophisticated internet media campaigns – thousands propaganda materials that were widely disseminated on the internet. Currently, however, there is very little empirical analysis of what was marketed in ISIS’s propaganda, what messages resonated with potential recruits, and what types of content were more likely to radicalize. This project breaks ground in research on internet propaganda and radicalization in several ways. Using video-as-data object detection and automated audio-to-text transcription algorithms, we uncover recruitment messages in propaganda produced by ISIS between January 1, 2015 and December 31, 2016. We identify the disseminations of these materials in a large dataset of Islamic State sympathizers on Twitter. Employing information on network connections, we find users who were exposed to propaganda, and study how exposure shaped their subsequent online behavior. Our findings show that propaganda content relating to grievances, ideology, and the material and social desires of potential recruits was highly effective at increasing online support for ISIS. Strikingly, however, these messages became largely ineffective when propaganda included violent imagery. These findings suggest that what attracted individuals to ISIS was not the brutal violence that made the group so famous, but the messages in its propaganda that conveyed the material, spiritual, and social benefits of recruitment.