Fort Delaware is located in the Delaware River on Pea Patch Island about forty-two miles downstream from Philadelphia. The island was first fortified with earthworks during the War of 1812. After a number of intermittent attempts to erect a permanent fort, Congress in 1847 appropriated $1 million to construct a permanent masonry fort on the site, and work began the following year. When the fort was finally completed in 1859, it had cost an additional million dollars. It was garrisoned by federal forces in 1861.
The fort was designed to mount 252 guns, and eventually 131 were installed. During the Civil War it saw use primarily as a prison for Confederate soldiers. The prisoners taken at the Battle of Gettysburg were held here. According to the 1870 Surgeon General's report on Fort Delaware, the prisoners of war "were confined in wooden barracks outside the fort," and "the greatest number imprisoned here at one time was about 12,000."
The Army maintained a garrison at the fort until its inactivation in 1870. The Spanish-American War and World War I provided impetus to reactivate Fort Delaware and modernize some of the gun emplacements, but it never fired a shot in anger. The state of Delaware assumed responsibility for the fort in 1944 and now maintains it as a tourist attraction.