Ordered to join the Pacific Squadron in 1854, the sloop of war Decatur sailed from Norfolk, Virginia, through the Strait of Magellan to Valparaiso, Honolulu, and Puget Sound, then on to San Francisco, Panama, Nicaragua, and Costa Rica, while serving in the Pacific until 1859, the eve of the Civil War. Historian Lorraine McConaghy presents the ship, its officers, and its crew in a vigorous, keenly rendered case study that illuminates the forces shaping America's antebellum navy and foreign policy in the Pacific, from Vancouver Island to Tierra del Fuego. One of only five ships in the squadron, the Decatur participated in numerous imperial adventures in the Far West, enforcing treaties, fighting Indians, suppressing vigilantes, and protecting commerce. With its graceful lines and towering white canvas sails, the ship patrolled the sandy border between ocean and land. Warship under Sail focuses on four episodes in the Decatur's Pacific Squadron mission: the harrowing journey from the Atlantic to the Pacific Ocean through the Strait of Magellan; a Seattle war story that contested American treaties and settlements; participation with other squadron ships on a U.S. State Department mission to Nicaragua; and more than a year spent anchored off Panama as a hospital ship. In a period of five years, more than 300 men lived aboard ship, leaving a rich record of logbooks, medical and punishment records, correspondence, personal journals, and drawings. Lorraine McConaghy has mined these records to offer a compelling social history of a warship under sail. Her research adds immeasurably to our understanding of the lives of ordinary men at sea and American expansionism in the antebellum Pacific West.