Fort Mifflin is one of the longest continually-operated military facilities in the United States. It was commissioned in 1771 to protect Philadelphia from attack or invasion. During the American Revolution it was the site of the largest bombardment of the War (November 10-16, 1777). The siege gave the Continental Army sufficient time to withdraw to Valley Forge, prolonging the Revolutionary War that ultimately gained America its independence. The Fort was reconstructed in 1797 as part of the First and Second American Systems of Fortification, a series of coastal defense networks designed to protect America's port cities. It later served as a Union Army prison during the Civil War and a munitions depot during World War I and II. Decommissioned in 1954, Fort Mifflin is now a National Historic Landmark, hosting guided tours, living history programs, and a variety of engaging events for school groups, scouts, and the general public. A number of Great War relics have been reclaimed and the Fort had an import part to play in the conflict.