The first full biography of Alfred Thayer Mahan in nearly fifty years explores how his groundbreaking ideas on sea power reshaped naval strategy and global politics--and why they still matter today. Drawing on rich archival material, David A. Smith brings to life the man whose intellect helped chart the course of modern maritime history. Alfred Thayer Mahan's theories of sea power transformed the strategic thinking of his time and continue to influence global affairs today. This is the definitive biography of the man whose writings reshaped naval doctrine and engendered a new understanding of international power. Author David A. Smith examines in full Mahan's intellectual development, professional path, and the broad consequences of his work for both the United States and the wider world. Chapters focus on the major stages of Mahan's life and thought: his early years as the son of a West Point professor, his formative experience at the U.S. Naval Academy, and his service during the Civil War, where he began to reflect on the operational and strategic dimensions of naval conflict. His time at the Naval War College marks a turning point where, distanced from command and engaged in study, he produced The Influence of Sea Power upon History, 1660-1783--a work that would revolutionize how nations conceived of maritime power and its role in national strength. Smith places Mahan's theories in the context of the industrial and imperial expansions of the late nineteenth century and tracks the reception of his ideas in Europe, Japan, and beyond. Mahan emerges here not simply as a theorist, but as a participant in the great debates of his age--about commerce, strategy, and the nature of national power. Drawing on personal correspondence, official records, and Mahan's own prolific writings, this biography also addresses the paradoxes of his character. He was an introvert in a profession that prized command, a strategist more at ease with books than with crews. His career unfolded in peacetime posts and classroom lectures rather than combat, yet his influence reached admirals, kings, and presidents. Above all, Mahan was a thinker whose work shaped modern navalism and whose legacy continues to inform discussions of military force and international order. This biography is a portrait of a mind that saw the ocean not as backdrop, but as the main arena in which nations rise and fall.