The War of the Pacific (1879-1884) is the war among South American states with the second highest casualty rate in the nineteenth century. This chapter provides a detailed case study of this war while offering a long-term narrative of state building in the South Pacific (i.e., Bolivia, Chile, and Peru). The comparison between Chile and Peru is illuminating, since both countries were comparable in important confounders–e.g., their armies, navy, bureaucracies, and budgets–and were impacted similarly by important economic confounders such as economic booms and crises. In this chapter I depict the evolution of war and the balance between central and peripheral elites from independence to the mid-century. Then I illustrate how preparation for war led to state formation, and looks at the details of the campaign, battle by battle. These two sections already serve the purpose of debunking some myths in this literature, like the idea that Peru did not mobilize for the war, and that the war did not lead to extraction in Chile Finally, I discuss how war transformed state institutions, and determined diverging, long-terms trends in state capacity.