Why do Westerners become foreign fighters in civil conflicts? We explore this question through original data collection on German foreign fighters in Syria and Iraq, and test three sets of hypotheses that revolve around socioeconomic integration, online radicalization, and social network mobilization. We conduct link analysis to map the network of German foreign fighters prior to their mobilization, and marshal evidence to assess the validity of competing explanations. We find only modest support for the integration deficit hypothesis, and meager support for the social media radicalization theory. Instead, the preponderance of evidence suggests that interpersonal ties largely drive the German foreign fighter phenomenon. Recruitment featured clustered mobilization and bloc recruitment within interconnected radical milieus, leading us to conclude that peer-to-peer networks are the most important mobilization factor for German foreign fighters.