Fort Mifflin, originally called Fort Island Battery and also known as Mud Island Fort, was commissioned in 1771 and sits on Mud Island (or Deep Water Island) on the Delaware River below Philadelphia, Pennsylvania near Philadelphia International Airport. During the American Revolutionary War, the British Army bombarded and captured the fort as part of their conquest of Philadelphia in autumn 1777. In 1795, the fort was renamed for Thomas Mifflin, a Continental Army officer and the first post-independence Pennsylvania governor. The U.S. Army began rebuilding the fort in 1794, and continued to garrison and build on the site into the 19th century. Fort Mifflin housed prisoners during the American Civil War. The U.S. Army decommissioned Fort Mifflin for active duty infantry and artillery in 1962. However, while the older portion of the fort was returned to the City of Philadelphia, a portion of the fort's grounds are still actively used by the U.S. Army Corps of Engineers, making it the oldest fort in use by the U.S. military. Historic preservationists have restored the fort, which has been named a National Historic Landmark. History Colonial defense of Philadelphia Built in 1681 in Philadelphia near the confluence of the Delaware and Schuylkill Rivers, Fort Mifflin was recognized as strategically important because of the role it played in defense of the settlement. However, William Penn, a Quaker with religious objections to military life, left Philadelphia undefended. When European colonists established permanent settlements, they also traditionally provided protection of those settlements, but Quakers founded the only significant European settlements without any such fortifications. Since the Quakers rejected the military, they instead sought to make peace with Native American tribes in the area and avoid any need to fortify their settlements militarily. While other colonies suffered from conflict and warfare, Philadelphia prospered. By the 1740s, Fort Mifflin ranked as the richest British port in the New World. French and Spanish privateers then entered the Delaware River, threatening the city. During King George's War (1744–1748), Benjamin Franklin raised a militia, because the (mostly Quaker) legislators of the city decided to take no action to defend Philadelphia "either by erecting fortifications or building Ships of War". He raised money to create earthwork defenses and to buy artillery. At the end of the war, commanders disbanded the militia and left derelict the defenses of the city. With renewed colonial warfare in the 1750s, especially the French and Indian