League Island was an island in the Delaware River, part of the city of Philadelphia, just upstream from the mouth of the Schuylkill River, which was the site of the Philadelphia shipyard, which eventually became the Philadelphia Naval Shipyard, now known as the Philadelphia Naval Business Center.
League Island is no longer actually an island, since the Back Channel, which separated it from the mainland, was closed at the eastern end to create land for a military airfield, Mustin Field, which is no longer existent. The western end of the channel became the present Reserve Basin, which once held the ships of the U.S. Navy's reserve fleet.