C. E. Callwell’s (1896; 1899; 1906) Small Wars: Their Principles and Practice was perhaps the most influential British Imperial irregular warfare manual of its time. Many existing accounts treat Callwell and other European colonial militarists as primitive—originals from which to explain later theorists’ descent and deviation. I show how he arrived at his account. Previous theorists, like Ewald, drew military lessons from early modern European irregular war, including targeted use of force by stealthy and mobile “light troops.” In contrast, Callwell advocated arbitrary and overwhelming violence, against combatants and civilians alike. I locate Callwell’s thinking at the end of the intellectual and political long nineteenth century. He exemplified a distinctively reactionary strand of British imperial thinking, imagining empire as permanent. His historical knowledge and field experience were encyclopedic. He linked a reactionary-utopian colonial nostalgia with systemic and racist high modernist violence. In the South African War (1899-1902), he helped deploy these practices against white Afrikaner colonists. His manual remained influential into the early twentieth century.