This dissertation examines naval policymaking in the United States and Britain in the era of the First World War, from the elevation of Admiral John Fisher to the position of First Sea Lord in 1904 to the end of the Washington Conference in 1922. Specifically, it analyzes how each country's navy developed policy and doctrine, and the ways in which institutional culture, strategic priorities, and administrative structure shaped these processes. The project explores these issues through both navies' experience with battlecruisers, a then-new type of large warship with heavy guns, high speed, and light armor. While doing so, the project also sheds light on the comparatively neglected American battlecruiser program, showing how crucial the ships were to American conceptions of future naval wars. Battlecruisers provide an ideal background for comparing British and American naval policy. The Royal Navy introduced the type, beginning construction on the first Invincible-class battlecruisers in 1905. On the other hand, the United States was the last major naval power to accept battlecruisers, and only started building them in 1916. These disparate stories allow us to see how each navy identified strategic priorities, allocated resources, developed doctrine, designed warships, and changed doctrine and design in response to technological developments and wartime experience. As the dissertation shows, the United States and the United Kingdom took very different approaches towards managing and maintaining sea power. Some of this was due to each country's national culture and strategic situation, but the institutional culture and administrative structure of each service played a role as well. In Britain, the need to defend a far-flung empire was filtered through the Admiralty, which could be dominated by the theories and passions of a single man. Across the Atlantic, the U.S. Navy's Mahanian worldview was constantly modified by the service's "strategic elite" in the Naval War College and on the General Board. These differences were reflected in each country's battlecruiser program: by the early 1920s, the Royal Navy built theirs for fighting battleships in major fleet actions, while the American battlecruisers were intended for scouting and long-range independent operations.