The British army under Major General William Howe occupied Philadelphia in late September 1777. Despite their success with occupying the American capital city, the British were in dire need of supplies — munitions, food and clothing. A huge fleet of ships laden with those necessities waited in the upper Delaware Bay, but was dissuaded from rendering aid because of two American forts on the Delaware River below Philadelphia.
Fort Mifflin was located near the mouth of the Schuylkill River on Mud Island. Fort Mercer occupied a position somewhat farther up the river on the New Jersey side. The American defenses included the deployment of cheveaux-de-frise (iron horses), which usually were constructed by driving 5-foot-long sharpened spikes through large timbers. These devices were towed into the river channel at strategic points and weighted down below the water level, where they served as impediments to enemy ship movements.
On October 22, American defenders at Fort Mercer under the command of Colonel Christopher Greene (cousin of Nathanael) withstood a fierce assault by Hessian mercenaries led by Colonel Carl Emil Kurt von Donop. Patriot sharpshooters took a heavy toll among the German officers and men. Donop had erred badly by attacking without a preparatory artillery barrage and was mortally wounded.
The British took at different approach at Fort Mifflin. Beginning in late October, they began a protracted bombardment of the American position. Some of the British ships had managed to get by the submerged iron horses with the aid of a spy within the American ranks. Efforts were made each night to repair the damage inflicted by the day’s shelling, but the American garrison was slowly being reduced. Following a major bombardment on November 10, Washington accepted his lieutenants' advice and sent word to evacuate the fort. That withdrawal took place on November 17.
The following day, Lord Charles Cornwallis landed 2,000 British troops on the New Jersey shore near Fort Mercer. The American commanders quickly decided to evacuate the garrison, torching the structures and all the supplies they could not carry away.
The fall of Forts Mifflin and Mercer assured that Howe and his officers could enjoy a warm winter in Philadelphia. Washington and his troops, however, were not so blessed and proceeded to winter quarters at Valley Forge.