This study retraces the origins of the modern art market to the First World War. While this transnational study touches upon processes of art politicization in an age of nationalism, it also examines cultural and social transformations within a European bourgeoisie threatened by revolutions, inflation, and financial crises. I argue that the war transformed an integrated art milieu, previously shaped by upper-class art patrons, into divided and highly nationalized art markets that were more strongly driven by capitalist incentives of investment and speculation, but also open for larger social strata. Scholarship on the art trade in the early twentieth century has largely been limited to national case studies and has focused on famous collectors, dealers and important artists who defined narrow segments of the market. By contrast, this dissertation explores price developments and mechanisms of supply and demand in wider segments of the auction market. The focus of this study is on Great Britain, France, and the German-speaking world, but it also includes sections on Russia, Italy, and Switzerland. In addition to sources such as auction reports, newspaper articles, and individual memoirs, my research is based on a large statistical index. This database contains the results of roughly 2600 auctions held between 1910 and 1925. It allows for making new observations about the evolution of European art markets, their disruption by the events of the First World War, and their transnational entanglements.