We sought to identify the mechanisms that cause strongly fused individuals (those who have a powerful, visceral feeling of oneness with the group) to make extreme sacrifices for their group. A large multinational study revealed a widespread tendency for fused individuals to endorse making extreme sacrifices for their country. Nevertheless, when asked which of several groups they were most inclined to die for, most participants favored relatively small groups, such as family, over a large and extended group, such as country (Study 1). To integrate these findings, we proposed that a common mechanism accounts for the willingness of fused people to die for smaller and larger groups. Specifically, when fused people perceive that group members share core characteristics, they are more likely to project familial ties common in smaller groups onto the extended group, and this enhances willingness to fight and die for the larger group. Consistent with this, encouraging fused persons to focus on shared core characteristics of members of their country increased their endorsement of making extreme sacrifices for their country. This pattern emerged whether the core characteristics were biological (Studies 2 and 3) or psychological (Studies 4–6) and whether participants were from China, India, the United States, or Spain. Further, priming shared core values increased the perception of familial ties among fused group members, which, in turn, mediated the influence of fusion on endorsement of extreme sacrifices for the country (Study 5). Study 6 replicated this moderated mediation effect whether the core characteristics were positive or negative. Apparently, for strongly fused persons, recognizing that other group members share core characteristics makes extended groups seem “family like” and worth dying for.