Ft. Mercer on the South Jersey side of the river compliments Mt. Mifflin. It, too, has a heroic history. These forts were originally built to defend Philadelphia from attack but after the British took Philadelphia, they blocked supplies from reaching Philadelphia. There was a third fort, never completed, further down river. The battle talked about was when the British sent a flotilla up the Delaware to force their way past the forts. They eventually succeeded but it was a bloody, prolonged battle. Some think that the artillery duel was more intense than at Yorktown. This battle also marked the end of Pennsylvania navy. Those ships not sunk by the British later fled up the river north of Philadelphia and were scuttled.