Despite great falls in global poverty, civil and ethnic conflict remains tragically common. In this chapter, I examine the patterns of persistence and change in conflict around the world through the lens of historical political economy. I compare two important approaches for measuring the impact of contemporary ethnic differences on conflict: ethno-linguistic fractionalization and polarization. I highlight two aspects which I believe are crucial for understanding these impacts but remain relatively understudied in this literature: the role of organizational capacity within groups and complementarities between groups. I illustrate these relationships through the history of Hindu-Muslim relations in South Asia, as well as drawing on broader contexts. Finally I highlight some common pitfalls that arise from neglecting history when studying conflict, and promising directions for future research.