Identity fusion is traditionally conceptualized as innately parochial, with fused actors motivated to commit acts of violence on out-groups. However, fusion’s aggressive outcomes are largely conditional on threat perception, with its effect on benign intergroup relationships underexplored. The present article outlines the fusion-secure base hypothesis, which argues that fusion may engender cooperative relationships with out-groups in the absence of out-group threat. Fusion is characterized by four principles, each of which allows a fused group to function as a secure base in which in-group members feel safe, agentic, and supported. This elicits a secure base schema, which increases the likelihood of fused actors interacting with out-groups and forming cooperative, reciprocal relationships. Out-group threat remains an important moderator, with its presence “flipping the switch” in fused actors and promoting a willingness to violently protect the group even at significant personal cost. Suggestions for future research are explored, including pathways to intergroup fusion.