Siege of Fort Mifflin (1777) – The Defense of Mud Island

Battle at a Glance

Historical Background

The British Capture of Philadelphia

On September 26, 1777, British troops under General Sir William Howe occupied Philadelphia. The city was the meeting place of the Continental Congress and an important political center of the American Revolution.

Although the British had captured Philadelphia, they had not destroyed Washington’s Continental Army. They also faced the challenge of supplying a large military force in a hostile region.

The Delaware River Supply Route

The Delaware River provided the most efficient route for transporting supplies into Philadelphia. British commanders needed to move food, ammunition, military equipment, and other necessities from ships operating in the lower river.

However, American defenses prevented British vessels from freely navigating the approaches to the city.

The American River Defense System

American commanders had constructed a defensive system consisting of forts, artillery batteries, naval forces, and submerged obstacles.

The principal fortifications included Fort Mifflin on Mud Island and Fort Mercer at Red Bank, New Jersey.

The Chevaux-de-Frise

Submerged timber structures armed with iron-tipped projections were placed in the Delaware River. These obstacles, called chevaux-de-frise, were designed to damage or obstruct vessels attempting to pass through the defended channels.

The obstacles forced British ships to navigate restricted passages that could be covered by American artillery.

Fort Mifflin on Mud Island

Fort Mifflin occupied a low island in the Delaware River south of Philadelphia. Its position allowed American artillery to threaten vessels attempting to pass through the river defenses.

The fort was unfinished and vulnerable to sustained bombardment, but its location made it strategically important.

Fort Mercer at Red Bank

Fort Mercer stood on the New Jersey shore opposite the defended river approaches. Its garrison helped protect the river obstacles and supported the broader defensive system.

On October 22, American troops under Christopher Greene defeated a Hessian assault against Fort Mercer.

American Naval Forces

Commodore John Hazelwood commanded American naval forces operating in the Delaware River. These included galleys and other vessels designed to support the river defenses and harass British shipping.

The naval forces worked in conjunction with the forts, although their operations were not always fully coordinated.

British Strategic Planning

Howe recognized that the American river defenses had to be eliminated before Philadelphia could be supplied efficiently by water.

British forces began preparing artillery positions on nearby islands and organizing naval operations against the American fortifications.

Province Island

Province Island lay near Mud Island and provided the British with ground suitable for constructing artillery batteries. British engineers gradually improved these positions to bring increasingly effective fire against Fort Mifflin.

The Beginning of the Siege

British pressure on the Delaware defenses began after the occupation of Philadelphia. The siege developed through a series of artillery exchanges, naval actions, engineering operations, and increasingly destructive bombardments.

The most intense phase occurred in November, when British batteries and warships concentrated their fire against the fort.

Commanders & Participating Forces

American Command Structure

Fort Mifflin’s garrison included Continental infantry, artillery personnel, engineers, and supporting troops. Command changed during the siege as officers were wounded or reassigned.

Lieutenant Colonel Samuel Smith

Samuel Smith was a Maryland Continental officer who commanded Fort Mifflin during an important phase of the defense. He directed the garrison while British forces increased their pressure on the fort.

Smith was wounded during the siege, contributing to changes in the fort’s command arrangements.

Major François-Louis Teissèdre de Fleury

Fleury was a French officer serving in the Continental Army. His engineering knowledge was valuable in maintaining and repairing the fortifications under bombardment.

He helped organize defensive work and contributed to the continued resistance of the garrison.

Major Simeon Thayer

Thayer was a Rhode Island Continental officer who assumed command during the later stages of the siege. He directed the defense as British artillery and naval fire increasingly destroyed the fort’s structures.

Thayer ultimately supervised the evacuation of the surviving garrison.

Continental Infantry

Infantry detachments defended the fort’s batteries, supported artillery operations, and helped repair damaged works. Their assignments changed as casualties accumulated and the siege progressed.

American Artillery

Artillerymen operated the fort’s cannons and attempted to suppress British batteries and naval vessels. They faced constant danger from incoming artillery fire.

American Engineers

Engineers and laboring soldiers worked to repair parapets, reinforce defensive structures, and maintain artillery positions.

These tasks were especially dangerous because repairs often had to be performed under enemy observation and fire.

Commodore John Hazelwood

Hazelwood commanded American naval forces operating in the Delaware River. His vessels supported the river defenses and attempted to interfere with British naval operations.

British Command Structure

General Sir William Howe

Howe directed the British occupation of Philadelphia and the military operations intended to secure the Delaware River supply route.

Vice Admiral Lord Richard Howe

Richard Howe commanded British naval forces supporting the Philadelphia Campaign. His ships attempted to overcome the American river defenses and assist the army’s siege operations.

Captain John Montresor

Montresor was a British military engineer involved in the Philadelphia Campaign. British engineering operations were essential to the construction of artillery batteries capable of damaging Fort Mifflin.

British Artillery

British artillerymen operated increasingly powerful batteries on Province Island. Their guns gradually damaged the fort’s earthworks, buildings, and artillery positions.

Royal Navy Warships

British warships provided additional firepower from the Delaware River. Their ability to approach the fort depended upon tides, channels, and the American river obstacles.

Comparing Strength

The American garrison consisted of several hundred soldiers, but its strength changed throughout the siege as reinforcements arrived and casualties accumulated.

The British possessed a much larger force in the Philadelphia region, although only portions of the army and navy participated directly in the bombardment and siege operations.

Battlefield Geography & Terrain

Mud Island

Fort Mifflin occupied Mud Island, a low-lying island in the Delaware River south of Philadelphia. Its location allowed American artillery to cover important navigation channels.

The Delaware River

The Delaware was the principal water route into Philadelphia. The river’s channels, tides, and submerged obstacles influenced the movement of British and American vessels.

Fort Mifflin’s Position

The fort was constructed on relatively low ground, making it vulnerable to artillery fire from nearby elevated or prepared positions.

Its strategic value came from its ability to threaten vessels attempting to pass through the river defenses.

Province Island

Province Island provided the British with ground from which artillery batteries could bombard Fort Mifflin. Engineers gradually improved these positions to increase the effectiveness of their fire.

Fort Mercer

Fort Mercer stood on the New Jersey shore and formed part of the same defensive system. Its artillery and infantry helped protect the river obstacles and support American control of the channel.

The Chevaux-de-Frise

Submerged timber structures armed with iron-tipped projections obstructed the river. These obstacles restricted British navigation and forced ships toward defended channels.

The River Channels

British naval operations depended upon the depth and location of navigable channels. Large warships could not move freely through shallow water or across the submerged obstacles.

The Fort’s Earthworks

Earthen parapets provided protection against artillery fire, but repeated bombardment gradually damaged the fortifications.

Wooden Defensive Structures

Some of the fort’s structures were constructed from timber and other materials vulnerable to artillery fire. These buildings and platforms suffered extensive damage during the siege.

The Modern Fort

Fort Mifflin survives as a historic site near Philadelphia International Airport. The present fortifications reflect later construction and modifications, so visitors should distinguish the surviving structures from the defenses that existed in 1777.

Weapons, Equipment & Logistics

American Artillery

The defenders operated cannons intended to threaten British vessels and land batteries. Their guns were positioned behind earthworks and other protective structures.

British Siege Artillery

British forces established artillery batteries on Province Island. Their guns were capable of firing sustained bombardments against the fort’s defensive structures.

Royal Navy Guns

British warships carried substantial artillery. Naval bombardment added another direction of attack and increased the pressure on the American garrison.

Mortars and Explosive Shells

Mortars could fire explosive shells over defensive walls and into the fort. Such weapons were particularly dangerous to soldiers sheltering behind parapets.

American Muskets

Continental infantry carried smoothbore muskets and bayonets. However, the siege was primarily an artillery engagement rather than a conventional infantry battle.

Artillery Ammunition

Both sides required substantial quantities of powder, shot, and other ammunition. The British possessed the logistical resources to sustain increasingly heavy bombardments.

Repairing the Fort

American soldiers repeatedly repaired damaged parapets and artillery positions. These repairs were essential to keeping the fort operational.

Disabled Guns

British fire damaged American artillery pieces and their platforms. The loss of guns reduced the defenders’ ability to answer the bombardment.

Fire and Structural Damage

Artillery strikes damaged wooden structures and created fires within the fort. The defenders had to contend with both incoming fire and the destruction of their own defensive facilities.

Medical Supplies

The garrison required medical treatment for soldiers wounded by artillery fire, flying debris, and collapsing structures.

Evacuation Equipment

The defenders ultimately needed boats to withdraw from Mud Island. The evacuation required coordination under dangerous conditions after much of the fort had been destroyed.

Intelligence & Operational Planning

Howe’s Logistical Problem

British occupation of Philadelphia required a secure supply route. American defenses prevented unrestricted naval access, forcing Howe to commit troops and artillery to the Delaware River campaign.

The British Siege Plan

British commanders intended to neutralize Fort Mifflin through artillery bombardment and naval operations while also attacking Fort Mercer and clearing the river obstacles.

Construction of Province Island Batteries

British engineers established artillery positions on Province Island. These batteries were gradually improved to increase their effectiveness against the fort.

American Defensive Preparations

The American garrison strengthened earthworks, maintained artillery, and attempted to preserve the fort’s ability to resist British attacks.

The Role of American Naval Forces

Hazelwood’s vessels supported the river defenses and attempted to interfere with British operations. Their activities complicated British efforts to approach the fort by water.

The Battle of Fort Mercer

On October 22, Hessian troops under Carl Emil von Donop attacked Fort Mercer at Red Bank. The Americans repelled the assault, temporarily preserving the New Jersey portion of the defensive system.

British Naval Losses

On October 23, British naval forces suffered serious losses during operations near the river defenses, including the destruction of HMS Augusta.

These losses demonstrated the hazards of operating large warships in restricted waters under enemy fire.

British Artillery Improvements

British engineers continued strengthening the batteries on Province Island. Their improved positions allowed increasingly destructive fire against Fort Mifflin.

The Final Bombardment

Beginning on November 10, British artillery and naval forces subjected the fort to sustained bombardment.

The defenders continued resisting despite extensive damage and casualties.

The Fort Becomes Untenable

By November 15, much of the fort’s defensive structure had been destroyed. The surviving garrison could no longer maintain an effective defense.

The Evacuation

During the night of November 15, American troops evacuated Mud Island. The British subsequently occupied the abandoned position.

Siege of Fort Mifflin: Interactive Timeline

Tactical Analysis & Command Decisions

Phase One: The Americans Defend the River

American commanders relied upon Fort Mifflin, Fort Mercer, naval forces, and submerged obstacles to obstruct British shipping.

The defensive system forced the British to undertake substantial operations before establishing a secure maritime supply route to Philadelphia.

Phase Two: British Artillery Positions Are Established

British engineers constructed batteries on Province Island. These positions allowed artillery to fire against Fort Mifflin from nearby ground.

Phase Three: The Fort Responds

American artillery attempted to suppress British batteries and protect the river defenses. The garrison also repaired damaged works and maintained the fort’s guns.

Phase Four: Naval Operations Intensify

Royal Navy vessels attempted to support the siege and clear the river. Their movements were complicated by shallow water, tides, and the American obstacles.

Phase Five: Fort Mercer Holds

The American victory at Fort Mercer on October 22 preserved the New Jersey portion of the defensive system and delayed British efforts to secure the river.

Phase Six: British Naval Losses

The destruction of HMS Augusta and HMS Merlin on October 23 demonstrated the dangers of operating warships near the American defenses.

Phase Seven: British Batteries Become More Effective

British engineers improved their artillery positions and increased the destructive effect of the bombardment.

The Americans struggled to repair the fort as quickly as the British could damage it.

Phase Eight: The November Bombardment

Beginning on November 10, British forces subjected the fort to sustained artillery fire. The bombardment damaged parapets, buildings, artillery platforms, and other defensive structures.

Phase Nine: The Defenders Continue Resistance

American soldiers continued operating guns and repairing damaged works despite casualties and deteriorating conditions.

Phase Ten: The Fort’s Defenses Collapse

By November 15, the fort had suffered extensive damage. British fire had disabled much of the defensive system and made continued resistance increasingly dangerous.

Phase Eleven: American Evacuation

The surviving garrison withdrew from Mud Island during the night of November 15. The British subsequently occupied the abandoned fort.

Conceptual Tactical Diagram

This schematic illustrates the relationship between Fort Mifflin, Province Island, Fort Mercer, and the Delaware River. It is not a surveyed reconstruction of exact 1777 positions.

Why Fort Mifflin Held for Weeks

The fort occupied a strategically important position supported by river obstacles, Fort Mercer, and American naval forces. The defenders repeatedly repaired damaged works and continued resisting despite British pressure.

Why the British Eventually Succeeded

British engineers gradually established artillery positions capable of delivering sustained destructive fire. Combined with naval bombardment, these batteries damaged the fort faster than the Americans could repair it.

The Limits of the British Victory

The British captured Fort Mifflin, but the operation required substantial time and resources. The delay complicated the supply of Philadelphia and demonstrated the effectiveness of the American river defense system.

The Soldier’s Experience

Life on Mud Island

American soldiers occupied a low island exposed to artillery fire and difficult environmental conditions. The fort was unfinished, and the garrison had to maintain defenses while preparing for British attacks.

Artillery Bombardment

British cannon fire struck the fort’s earthworks, buildings, and artillery positions. Soldiers faced danger from direct hits, flying debris, and collapsing structures.

Repairing the Defenses

American soldiers repeatedly repaired damaged parapets and platforms. These tasks often had to be performed while the British continued firing.

Operating the Guns

Artillerymen worked under dangerous conditions to maintain defensive fire. Their guns were essential to the fort’s ability to resist British ships and land batteries.

Smoke and Dust

Repeated artillery fire filled the fort with smoke, dust, and debris. Visibility could be reduced as structures collapsed and fires developed.

Medical Conditions

Wounded soldiers required treatment for artillery injuries, burns, and trauma caused by flying fragments or collapsing structures.

Loss of Defensive Structures

As the siege continued, the fort’s buildings and artillery platforms suffered increasing damage. The defenders had fewer secure places to shelter from bombardment.

The Final Bombardment

Beginning on November 10, the British subjected Fort Mifflin to sustained fire. The bombardment continued for several days and caused extensive destruction.

The Last Day

By November 15, the fort’s defenses had been severely damaged. The surviving soldiers faced the possibility of remaining in a position that could no longer be defended effectively.

The Evacuation

During the night of November 15, the garrison withdrew from Mud Island by boat. The evacuation ended weeks of resistance.

After the Siege

The surviving defenders rejoined American forces. Their resistance had delayed British efforts to secure the Delaware River, even though the fort was ultimately lost.

Where Was Your Ancestor’s Unit?

The Siege of Fort Mifflin involved Continental infantry, artillery personnel, engineers, naval forces, and British siege troops. Because the garrison changed during the siege, researchers must identify the dates on which an ancestor served at the fort.

Principal Formations

Maryland Continental Troops

Samuel Smith was a Maryland Continental officer. Researchers should examine his correspondence and the records of troops assigned to the fort during his command.

Rhode Island Continental Troops

Simeon Thayer was a Rhode Island officer who commanded during the final defense. His service records and correspondence may help identify troops present during the November bombardment.

Continental Artillery

Artillerymen were central to the defense. Researchers should identify the specific artillery companies and detachments assigned to Fort Mifflin.

Continental Engineers

Engineering personnel helped maintain and repair the fortifications. Their work was essential to the fort’s ability to resist prolonged bombardment.

American Naval Personnel

Naval forces under John Hazelwood supported the river defenses. Their service should be distinguished from that of soldiers stationed inside Fort Mifflin.

British Artillery

British artillerymen operated the siege batteries on Province Island. Their records may identify the companies and guns involved in the bombardment.

Royal Navy Sailors

Royal Navy personnel served aboard warships participating in the river operations. Ship muster books and logs may provide evidence of individual service.

American Casualty Records

The siege produced casualties over several weeks. Researchers should distinguish men wounded during the earlier bombardments from those injured during the final November attack.

Revolutionary War Pension Applications

Veterans may describe service at Fort Mifflin, Mud Island, Fort Island Battery, or the Delaware River defenses.

Some pension applicants may refer to service under Samuel Smith, Simeon Thayer, or other officers rather than naming the siege directly.

Prisoners and Missing Soldiers

Casualty and prisoner records may identify soldiers who were wounded, captured, or missing during the siege and evacuation.

Distinguishing Fort Mifflin from Fort Mercer

Fort Mifflin stood on Mud Island, while Fort Mercer occupied the New Jersey shore at Red Bank.

The two forts formed part of the same defensive system but experienced different engagements.

Common Research Errors

Do not confuse Fort Mifflin with Fort Mercer.

Do not assume every soldier serving in the Delaware River defenses was stationed inside Fort Mifflin.

Do not assign all Maryland Continental troops to Samuel Smith’s garrison.

Do not assume every Rhode Island soldier served under Simeon Thayer at Mud Island.

Do not combine naval casualties from the wider Delaware River campaign with the fort’s garrison casualties.

Do not treat the entire siege as a single-day engagement.

Do not assume every soldier listed in a regimental return was present during the final bombardment.

Battle Outcome & Casualties

British Victory

The siege ended when the American garrison evacuated Fort Mifflin during the night of November 15, 1777. British troops subsequently occupied the abandoned position.

American Casualties

American losses accumulated throughout the siege, particularly during the final bombardment. Published accounts frequently describe casualties in the hundreds, but exact totals vary depending upon the dates and operations included.

British Casualties

British land and naval forces suffered casualties during the siege and associated operations. However, the broader Delaware River campaign included naval losses that should not automatically be counted as casualties of the Fort Mifflin garrison battle.

Destruction of the Fort

British artillery and naval fire severely damaged the fort’s defensive structures. By November 15, the position could no longer be defended effectively.

The American Evacuation

The surviving garrison withdrew by boat during the night. The evacuation preserved some of the remaining soldiers while ending the defense of Mud Island.

British Control of the Position

The British occupied Fort Mifflin after the Americans withdrew. Its capture weakened the remaining American defenses along the Delaware River.

The Meaning of the Result

The British ultimately captured Fort Mifflin, but the siege required weeks of artillery and naval operations. The American defenders delayed the opening of the Delaware River and imposed substantial costs upon British forces.

The battle illustrates how a relatively small garrison could influence a larger campaign by defending a strategically important position.

Strategic Aftermath

The British Occupy Fort Mifflin

After the American evacuation, British troops occupied the ruined fort. Its capture removed an important obstacle to British control of the Delaware River.

Fort Mercer Becomes Vulnerable

With Fort Mifflin abandoned, the American position at Fort Mercer became increasingly difficult to defend.

The Evacuation of Fort Mercer

On November 20, American forces evacuated Fort Mercer as British operations threatened the New Jersey shore position.

The Delaware River Opens

The removal of the American forts and river defenses allowed British forces to improve their naval supply route to Philadelphia.

British Supply Operations

With the river increasingly secure, British vessels could transport supplies more effectively to the occupying army.

Washington’s Army

Washington’s Continental Army remained active outside Philadelphia. The British had captured the city and secured its supply route, but they had not destroyed American resistance.

Valley Forge

In December 1777, Washington’s army entered winter quarters at Valley Forge. The encampment followed months of campaigning that included Brandywine, Paoli, Germantown, and the Delaware River defenses.

The French Alliance

In February 1778, France entered into formal treaties with the United States. The American victory at Saratoga had been the decisive military development encouraging French intervention.

The British Evacuate Philadelphia

In June 1778, British forces abandoned Philadelphia and marched across New Jersey. Washington pursued, leading to the Battle of Monmouth.

Fort Mifflin’s Later History

Fort Mifflin was rebuilt and modified after the Revolutionary War. The site continued to serve military purposes during later periods of American history.

Preservation of the Fort

Fort Mifflin survives as a historic site on the Delaware River. Its preserved structures and interpretation help visitors understand the importance of the 1777 defense.

The Legacy of the Siege

The Siege of Fort Mifflin demonstrates the strategic importance of fortifications, artillery, and river control during the Revolutionary War.

Although the Americans ultimately evacuated the fort, their resistance delayed British efforts to supply Philadelphia and became one of the most determined defensive operations of the Philadelphia Campaign.

Primary Sources & Historical Research

The Siege of Fort Mifflin is documented through American military correspondence, British engineering journals, Royal Navy records, artillery returns, and contemporary reports. Important research questions concern the changing composition of the garrison, the construction of British batteries, the chronology of the bombardment, and the casualties sustained during the final defense.

Primary Sources

1. Samuel Smith’s correspondence and reports, 1777. Evidence concerning the early defense of Fort Mifflin.
2. François-Louis Teissèdre de Fleury’s correspondence. Evidence concerning engineering and defensive operations.
3. Simeon Thayer’s reports. Evidence concerning the final defense and evacuation.
4. George Washington’s correspondence. Orders and reports concerning the Delaware River defenses.
5. General Sir William Howe’s correspondence. British perspective on the siege and the supply of Philadelphia.
6. John Montresor’s engineering records. Evidence concerning British artillery positions and siege operations.
7. Royal Navy logs and reports. Evidence concerning naval bombardments and river operations.
8. Continental artillery returns. Evidence concerning guns, ammunition, and artillery personnel.
9. Revolutionary War pension applications. Later testimony concerning individual service at Mud Island.

Historical Studies

1. Thomas J. McGuire, The Philadelphia Campaign, Volume Two: Germantown and the Roads to Valley Forge. Detailed study of the Philadelphia Campaign and Delaware River operations.
2. John W. Jackson, With the British Army in Philadelphia, 1777–1778. British military operations and occupation.
3. Christopher Ward, The War of the Revolution. Broader military history of the Revolutionary War.
4. Fort Mifflin historical and archaeological studies. Research concerning the fort’s construction, siege, and later modifications.
5. Delaware River defense studies. Research concerning Fort Mercer, naval operations, and the chevaux-de-frise.

Online Historical Resources

1. Fort Mifflin on the Delaware. Historic fort and interpretation of the 1777 siege.
2. American Battlefield Trust. Revolutionary War battlefield research.
3. Founders Online — National Archives. Washington’s correspondence and founding-era military records.
4. National Archives — Revolutionary War Records. Military service and pension research.
5. Library of Congress — Historical Maps. Historic maps of the Delaware River defenses.
6. Pennsylvania State Archives. Pennsylvania military and historical records.
7. The National Archives, United Kingdom. British military and naval records.

Research Notes and Historical Cautions

Siege dates: September 26–November 15, 1777.

Principal bombardment: November 10–15.

Location: Mud Island, Delaware River, Pennsylvania.

Contemporary name: Fort Island Battery.

Campaign: Philadelphia Campaign and Delaware River defense.

American commanders: Samuel Smith, François de Fleury, Simeon Thayer.

American naval commander: John Hazelwood.

British military commander: William Howe.

British naval commander: Richard Howe.

Principal British artillery position: Province Island.

American objective: Defend the river obstacles and prevent unrestricted British naval access.

British objective: Neutralize Fort Mifflin and open the Delaware River supply route.

Principal American forces: Continental infantry, artillery, engineers, and supporting naval forces.

Principal British forces: Siege artillery, infantry support, and Royal Navy warships.

Result: British victory following American evacuation.

Fort evacuated: Night of November 15.

Fort Mercer evacuated: November 20.

Historical caution: Casualty totals vary depending upon the dates and associated operations included.

Genealogical caution: The garrison changed during the siege; participation should be tied to specific dates.

Strategic context: The siege formed part of the British effort to secure the Delaware River following the capture of Philadelphia.