War experience showed that masonry forts were vulnerable to modern rifled guns. As a result, in 1869 construction at Fort Popham stopped before the fortification was completed. The fort was garrisoned again after additional work was performed during the Spanish–American War and World War I. In the 1890s Fort Popham received new facilities for a controlled minefield in the river at the fort as part of the Endicott program of improved fortifications. In 1898, shortly after the Spanish–American War broke out, Congress made numerous emergency appropriations, including $3,200 to deploy the mines in the Kennebec. In 1899, following the war, a single 8-inch M1888 gun was mounted near the fort on a converted Rodman carriage, joining four 15-inch Rodman guns and the 100-pounder Parrott rifle that remains near the fort; the 8-inch gun was removed in 1910. This was an emergency measure to provide modern guns at threatened locations until the Endicott program forts could be completed. Under this program, construction of Fort Baldwin on the headland above Fort Popham began in 1905 with longer-range guns, which eventually rendered Fort Popham obsolete. However, at the time Fort Baldwin was built, Fort Popham remained important due to the minefield facilities.