Standing as a formidable sentinel on the Delaware River, Fort Mifflin holds a pivotal place in American history, particularly during the Revolutionary War. Originally known as Fort Island Battery or Mud Island Fort, this historic fortification in Philadelphia, Pennsylvania, played a crucial role in defending the revolutionary capital. The dramatic story of Fort Mifflin encompasses strategic defenses, a harrowing siege, and an enduring legacy that continues to captivate historians and visitors alike.
Philadelphia’s strategic importance as the wealthiest British port in the New World by the 1740s necessitated robust defenses. During King George’s War (1744-1748), the threat of French and Spanish privateers prompted Benjamin Franklin to raise a militia for city defense, as the predominantly Quaker legislature opposed military engagement. Franklin’s efforts funded temporary earthwork defenses and artillery, but these were dismantled post-war. The renewed conflicts of the 1750s, notably the French and Indian War, again brought plans for a fort on Mud Island to the forefront, though construction did not commence at that time.
The Birth and Revolutionary Role of Fort Mifflin
By 1771, Philadelphia had grown to become North America’s largest British port, leading to increased tensions with British economic policies. Recognizing the need for permanent defenses, Pennsylvania Governor John Penn sought expert assistance from General Thomas Gage. Captain John Montresor of the British Corps of Engineers was tasked with designing fortifications for Mud Island (also known as Deep Water Island) to control river traffic. Montresor presented several designs, with his preferred plan envisioning a substantial fort mounting 32 cannon and requiring a garrison of 400 men, estimated at £40,000. Despite Montresor’s protests about inadequate funding, the Colonial Provincial Assembly approved £15,000 for construction and the purchase of Mud Island, a sum Montresor deemed insufficient. Construction began in 1772, but Montresor, disgruntled by the budget constraints, left the project, which subsequently floundered for a year, completing only the stone east and south walls.
Following the adoption of the Declaration of Independence, Benjamin Franklin chaired a committee to bolster Philadelphia’s defenses. This led to the resumption of construction on Fort Mifflin, which was finally completed in 1776. Concurrently, Fort Mercer was erected on the eastern bank of the Delaware River in New Jersey, directly across from Fort Mifflin. These two forts were strategically positioned to control British naval activity on the Delaware River, safeguarding Philadelphia against General Howe and Admiral Reynolds’ fleet.
To further impede British advance, an intricate system of river obstructions, known as chevaux de frise, was installed between Forts Mercer and Mifflin. These defenses comprised large wooden frames, ballasted with tons of stone and fitted with iron-tipped timbers submerged underwater, facing downstream. Chained together, these formidable barriers could impale unwary ships, with only a select few patriot navigators knowing the safe passages. Soldiers at both forts could unleash fire upon any British attempts to dismantle these obstructions.
The Pivotal Siege of 1777
In September 1777, following General George Washington’s defeat at the Battle of Brandywine, British forces occupied Philadelphia. To establish a crucial supply line via the Delaware River, the British launched a siege against Fort Mifflin and Fort Mercer in early October. Though the land and river assault on Fort Mercer at the Battle of Red Bank on October 22 was unsuccessful, the British intensified their focus on Fort Mifflin.
Ironically, Captain John Montresor, the fort’s initial designer, now led the British siege, planning and constructing the siege works that would devastate the fort. For nearly five weeks, approximately 400 American soldiers valiantly held off over 2,000 British troops and 250 ships. Private Joseph Plumb Martin, a defender during the siege, later chronicled the harrowing experience. Commodore John Hazelwood, commanding a significant American fleet, consistently harassed British river operations, launching raids and patrolling the waters around the fort.
On November 10, 1777, the British intensified their bombardment, unleashing a relentless barrage of cannonballs. Enduring the heaviest bombardment of the American Revolution, the fort suffered immense damage, with 250 of its more than 400 defenders killed or wounded. On November 15, 1777, the surviving American troops were evacuated. Despite its fall, the heroic stand at Fort Mifflin proved a strategic success, denying the British Navy free use of the Delaware River long enough for the Continental Army to reposition for the Battle of White Marsh and subsequent withdrawal to Valley Forge. After this critical engagement, Fort Mifflin never again saw military action during the Revolution.
Reconstruction and 19th-Century Evolution
The ruins of Fort Mifflin lay abandoned until 1793, when reconstruction began as part of the “first system” of U.S. coastal fortifications. Pierre L’Enfant, renowned for planning Washington, D.C., oversaw the initial design and rebuilding in 1794. From 1795 to 1800, engineer Louis de Tousard continued the reconstruction, aiming to accommodate 48 guns. In 1795, the fort was officially named after Major General Thomas Mifflin, a Continental Army officer and Pennsylvania’s first post-independence governor.
During this period, new structures emerged, including two-story barracks designed to house 25 men per room. In 1796, the extant citadel, now housing the commandant, was built over a cross-shaped defensive area. Lieutenant Colonel Stephen Rochefontaine, who replaced L’Enfant as chief engineer in 1798, completed the commandant’s house, a rare example of Greek Revival architecture on U.S. Army installations. From 1798 to 1801, six cavelike casemates were added as bomb-proof defensive structures, along with a bake oven near the main gate that served as a chapel and mess hall. By 1800, with the U.S. Congress meeting in Philadelphia, Fort Mifflin was well-garrisoned. The blacksmith shop, built before 1802, is believed to be the oldest surviving complete structure at the fort.
Repairs and upgrades continued into the 19th century, with the fort being described in the 1811 annual report as mounting 29 heavy guns and a water battery with eight heavy guns, alongside barracks for 100 men and a nearby lazaretto capable of housing 400 more. An officers’ quarters was built in 1814, and a guardhouse and prison in 1815-1816. By 1819, a two-story hospital and mess hall building was constructed. The area around the fort, originally Mud Island, was drained and filled during the 19th century, eventually connecting it to the western bank of the Delaware River.
The Civil War and Beyond
Despite the construction of Fort Delaware in 1820 relegating Fort Mifflin to secondary status, building and garrisoning continued, with meteorological observations commencing in the early 1820s. Extensive renovations occurred in 1836 for the barracks and officers’ quarters. The hospital and mess hall transformed into a meetinghouse in 1837, and an artillery shed was added. The guardhouse prison was designated an arsenal by 1839, and a two-story kitchen wing was added to the officers’ quarters in the 1840s.
During the American Civil War (1863-1865), Fort Mifflin gained notoriety as a prison. It housed Confederate prisoners of war in Casemate #1 and Union soldiers and civilian prisoners in three smaller casemates. The fort, with a maximum capacity of 250, housed up to 215 prisoners at one time, recording 42 escapes and three deaths. The most famous prisoner was William H. Howe, a Union soldier accused of desertion and found guilty of murder. Howe, known for his illiteracy, attempted an escape with 200 prisoners in 1864 and was subsequently held in solitary confinement in Casemate #11, where he left his signature. Despite his reputation, he penned two grammatically flawed letters to President Abraham Lincoln seeking clemency. Howe was executed by hanging at Fort Mifflin on November 24, 1864, before a paying audience – a unique occurrence among the fort’s executions.
In late 1864, Lieutenant Colonel Seth Eastman, a noted Western frontier painter, supervised the discharge of over 200 prisoners. After the Civil War, the fort underwent intermittent repairs and modernization by the Corps of Engineers between 1866 and 1876, including efforts on a detached high battery and a torpedo casemate. Casemate #11 today preserves a fascinating trove of historical artifacts, from pottery to period buttons and animal bones, offering a glimpse into its past.
A National Historic Landmark
From 1876 to 1884, the Philadelphia District Office of the Corps of Engineers held custodial responsibility for Fort Mifflin. Its role evolved, serving as a munitions storage facility during World War I. Disarmed and partially dismantled in 1904, the fort earned its designation as a National Historic Landmark in 1915 and was restored to its original plans in 1930.
During World War II, the fort returned to military service, with anti-aircraft guns stationed there to defend the nearby Naval Ammunition Storage Depot and the Philadelphia Naval Shipyard. Notably, Battery H of the 76th Coast Artillery Regiment, the first African American Coast Artillery unit in U.S. military history, was stationed at Fort Mifflin in April 1942. After their deployment overseas, the 601st Coast Artillery Regiment took over the fort’s defense.
Fort Mifflin was officially decommissioned as an active military post in 1954 and as an active duty infantry and artillery post in 1962, closing its long and distinguished chapter as one of the nation’s oldest continuously used forts. The fort’s interior underwent significant renovation in 1960. Today, Fort Mifflin stands as a testament to American resilience and military engineering, offering visitors a profound connection to over two centuries of history, from its pivotal role in the American Revolution to its service in two World Wars. It remains a cherished historic landmark, preserving the stories of those who defended Philadelphia and the nation.
