This paper seeks to answer the titular question of what drove commodity market integration in the 19th century. Using grain markets during the first wave of globalization as a testing ground, the paper builds on the insights of the contemporary trade literature and the economic history of the 19th century and relates levels of market integration to cross-sectional and temporal variations in transport technology, geography, monetary regimes, commercial networks/policy, and conflict. The results of this decomposition analysis are interesting on two counts: first, they verify the commonality of experience of the 19th and late 20th centuries; second, they suggest a very strong role for the commercial, diplomatic, and monetary environment in which market integration took place.