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The Fort on Mud Island, 1777.

By the mid-1700s Philadelphia was the largest and most important British port in North America. To protect city from a naval assault up the Delaware River, the British in 1772 funded the construction of a fort on Great Mud Island, just below the entrance to the Schuylkill River, and commissioned military engineer John Montresor to lay out its design.  The fort itself, however, was still unbuilt when Pennsylvania joined twelve other colonies in their war for independence.

Hessian Map of Delaware River Defenses, 1777.

To protect their capital from British invasion during the Revolutionary War, Congress in 1776 ordered elaborate defenses of the Delaware River, including chevaux-de-frise, iron-tipped logs embedded underwater to impale ships navigating upstream. To prevent enemy ships from clearing these obstacles, the Americans also completed the fort on Mud Island, and built Fort Mercer on Red Bank Island directly across the main channel on the New Jersey side of the river and additional defenses at Billingsport, on the Jersey shoreline.

Model of Chevaux-de-Frise as they would have looked in 1777.

When British Admiral Richard Howe departed Sandy Hook, New Jersey on July 23, 1777 in route to Philadelphia, he planned to sail his fleet of 260 vessels up the Delaware to attack Philadelphia directly. The formidable river defenses, however, convinced Howe to bypass the Delaware River and, instead, to sail south to Virginia and then north up the Chesapeake Bay. Landing in northern Maryland at the head of the Elk River, the British Army would then have a relatively short, fifty-mile march to reach Philadelphia.

Plan of Assault on Fort Red Bank on October 22, and Fort Mifflin on Mud Island...

Fort Mifflin, by Seth Eastman, 1870.

After driving back the Americans at the Battle of Brandywine, General William Howe and the British Army entered Philadelphia on September 26, 1777.  In order to maintain their supply lines and control of the city, they needed to secure the Delaware River, which provided the most direct access to the Atlantic Ocean. And to do this they needed to take control of Mud Island Fort and Fort Mercer, both still under American control.   

 On September 26, the British began their siege of Mud Island Fort.  Over the next three weeks, more than 200 British cannon pounded the fort in what became one of the greatest bombardments of the Revolutionary War.  After three weeks of shelling and a surprisingly strong defense, the Americans abandoned the fort on November 15.  By then, 250 of the more than 400 defenders had been killed or wounded. When Americans defenders left Fort Mercer soon afterwards, the British at last controlled the Delaware. Unfortunately for General Howe, however, the prolonged American defense had delayed the arrival of supply ships long enough to prevent any attack upon Washington's Continental Army until the following spring.

Rebuilt in the 1790s, the fort on Mud Island, renamed Fort Mifflin in 1795, continued as a military post during the War of 1812 and the Civil War, when it was used as a prison camp. Disarmed in 1904, the Fort was declared a National Monument by the United States Congress in 1906.  In the decades that followed, the Fort fell into further disrepair, until control of the site was given to the city of Philadelphia, which undertook an ambitious restoration and opened it to the public. Designated a National Historic Landmark in 1970, Fort Mifflin is now maintained as a historic site by the Philadelphia Department of Recreation.

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