The siege of Fort Mifflin or the siege of Mud Island Fort, which took place from September 26 to November 16, 1777, saw British land batteries commanded by Captain John Montresor and a British naval squadron under Vice Admiral Lord Richard Howe attempt to capture an American fort in the Delaware River that was commanded by Lieutenant Colonel Samuel Smith. The operation finally succeeded after Smith was wounded. His successor, Major Simeon Thayer, subsequently evacuated the fort on the night of November 15, enabling British troops to occupy the place the following morning.

Simeon Thayer fought in Rogers' Rangers during the French and Indian War and made a harrowing escape from French-allied Indians. At the outbreak of the American Revolutionary War, the Rhode Island assembly appointed him an officer. He quickly raised a company of soldiers and marched with them to the Siege of Boston. He and his men went on Benedict Arnold's expedition to Quebec during which time he kept a journal of his experiences. He fought at Quebec and was captured. After being paroled, he again served as an officer in the Continental Army. Simeon Thayer was a member of the Society of the Cincinnati of the State of Rhode Island.