We run an experiment to investigate how ideology, exemplified by differing opinions on a specific social topic, shapes group identity and influences behavior in conflict scenarios. Participants engaged in the Peacemaker Game, where disputants decide to cooperate or defect, and a third party chooses whether to intervene. The experimental design includes an opinion-divergence treatment, a minimal grouping treatment and a control condition. Results reveal that "opinion-based group identity" significantly influences in group favoritism, out group bias, and the likelihood of third party intervention. The findings underscore the substantial impact of ideological alignment on social interactions and provide valuable implications for understanding and mitigating conflicts in real-world scenarios marked by increasing polarization.