Construction of Fort Popham began in 1861 to guard the entrance to the Kennebec River and protect the shipbuilding interests in the upriver city of Bath as well as the state capital in Augusta. Although uncompleted, the Civil War era fort is an important example of military engineering from that period. After the State of Maine purchased Fort Popham from the United States Government in 1924, the structure became unstable over time and required significant renovations to remain safe and reopen closed areas while accommodating nearly 100,000 visitors each year. Its significance was recognized when it was listed in the National Register of Historic Places on October 1, 1969.