The war demonstrated that the fort's brick structure and high walls were no match for improved cannon technology (rifled guns). As a result, its worth as a fort was greatly diminished. That said, the army still used it for a variety of purposes including as army barracks, housing for single military officers, and trade schools. During World War II, the fort was used to guard the entrance to San Francisco Bay from submarine incursions. It was also at this time that efforts to preserve the fort began. It took a long time for the fort to be protected, however. It became a national historic site in 1970.