This chapter examines Callwell’s contribution to the ‘naval school’ that emerged as a result of increased interest in matters of imperial defence in the late nineteenth and early twentieth centuries. It argues that Callwell’s work was distinctive, based on his position as a ‘khaki-clad maritime theorist’, a soldier looking at amphibious warfare from the landsman’s perspective. It also explores Callwell’s time at the War Office, 1903-7, during which he was a practitioner as well as a theorist of strategy. Finally, it assesses Callwell’s literary output during his retirement, especially his writing on the Territorial Army.