The joint efforts of Roosevelt and Mahan to secure U.S. prominence in the Western Hemisphere through sea power and to develop a navy capable of confronting powerful enemies in the Atlantic and Pacific altered hemispheric relations irrevocably and had important consequences for the conduct and outcome of both world wars. The author describes the working relationship that developed between the two men when Roosevelt served first as civil service commissioner and subsequently as assistant secretary of the navy, and the events, controversies, and policies that eventually led to the creation of...