Pea Patch Island and Fort Delaware was declared surplus land by the United States Department of Defense in 1945.

The island also provides a significant wetlands stop for migratory birds including herons along the Atlantic Flyway.

Construction began on a much larger polygonal fort in 1836, but this project was derailed by a decade-long legal battle over which state owned the island, which was won by Delaware.

[3][4] During the American Civil War, Fort Delaware was used by the Union as a camp for Confederate prisoners, in particular those captured in 1863 at the Battle of Gettysburg.

After the release of the last of the remaining Civil War prisoners, only a small caretaker force was left behind at Fort Delaware, and it was largely abandoned in 1870.

The defenses around Pea Patch Island were disarmed during World War II, as Fort Miles at the mouth of the Delaware estuary superseded them.

[7] A group of reenactors pays special attention to Captain George Ahl and his band of former confederate soldiers who formed the 1st Delaware Heavy Artillery.

[9] Fort Delaware State Park is home to a migratory bird rookery, considered to be the largest heronry north of Florida.

[1] Ornithologists believe that ibises, egrets, and herons began nesting on the northern part of Pea Patch Island in the 1950s or 1960s on land that was deposited there by the United States Army Corps of Engineers in the early 1900s.

[1] Studies have shown that nearly half the chicks born at the heronry within the last five years have died before they were old enough to leave their parents care.

Representatives from local, state and federal governments have teamed together with non-profit wildlife organizations, business, and industry to create a Special Area Management Plan to help change the downward trend in bird populations at Fort Delaware State Park.

Since Beach erosion affecting Pea Patch Island was recognized as a potential threat to the Fort in 1999, the United States Army Corps of Engineers erected a 3,500 feet (1.1 km) seawall during the winter of 2005-2006.