“Under Siege: Kentucky and the Transformation of American Thoroughbred Racing 1865-1936.” Register KHS 118 (Summer 2020), 389-446. This article examines the transformation of Thoroughbred racing in the United States, the first mass spectator sport in America, from a provincial pastime to a national industry. Cycles of decline and resurgence of the sport were associated with changes in the social, political and economic fabric of the nation and by the upheaval of war. In association with these root causes, the geographic centers for breeding and racing shifted back and forth from east to west and south to north. Nearly extinguished during the Civil War, racing afterward became increasingly democratic and commercialized, and gambling became the most important revenue stream and a driving force in the post-war expansion of the industry. As the nineteenth century ended, the sport was roiled by competing internal factions and assaulted by Progressive anti-gambling social reformers who nearly succeeding in eliminating the sport. As a primary center for the racing industry, Kentucky provides a case study to examine changes on the broader national scale.