Fort DuPont, named in honor of Rear Admiral Samuel Francis Du Pont, is located between the original Delaware City and the modern Chesapeake and Delaware Canal on the original Reeden Point tract, which was granted to Henry Ward in 1675. Along with two other forts of the Harbor Defenses of the Delaware, it defended the Delaware River and the water approach to Philadelphia from 1900 through 1942. In 2016, the acreage which is not in the state park system was annexed into Delaware City.

The first fortification built was the Ten Gun Battery, an auxiliary to nearby Fort Delaware during the American Civil War. The Twenty Gun Battery was constructed on the reservation during the 1870s, later followed by a mine control casemate for an underwater minefield in 1876. In 1897-1904, Endicott-era emplacements were constructed for long-range rifles, mortars, and rapid-fire guns. In 1922 the post became headquarters for the 1st Engineer Regiment, which remained at the post until 1941. During World War II, Fort DuPont served as a mobilization station for deploying units, and contained a prisoner-of-war camp for captured German soldiers and sailors. After the war, Fort DuPont was declared surplus and offered to the Veterans Administration for use as a veterans hospital. After they declined, the state bought the site at a 100 percent discount and adapted existing structures for reuse. In 1948, it officially opened as the Governor Walter W. Bacon Health Center. In 1992 a portion was redesignated as Fort DuPont State Park, which became Delaware's 13th state park. In 1999 the site was officially designated the Fort DuPont Historic District after it was listed in the National Register of Historic Places. The historic district comprises Fort DuPont State Park and the Governor Bacon Health Center. The site is currently being redeveloped by the Fort DuPont Redevelopment and Preservation Corporation.

Civil War through 1885
Ten Gun Battery, briefly called Fort Reynolds, was built from 1863 to 1864 on the property of 1st Lt. Clement Reeves of the 5th Delaware Volunteer Infantry. The first soldiers to garrison the post were from Capt. John Jay Young's Independent Battery G, also called the Pittsburgh Heavy Artillery. Sgt. Bishop Crumrine of Young's Battery wrote, "This fortification is not properly a Fort but rather a water battery. Situated just across the river from Fort Delaware on the Delaware City side, it has five sides. The two longest sides being next to the river is a heavy breast work on which six 10-inch and four 15-inch Rodman guns are mounted." The battery was rebuilt as the Twenty Gun Battery in the 1870s. It was to house both heavy guns and coast defense mortars, but was not fully armed. In 1876 a mine casemate was built for an underwater minefield.

1885 through Spanish–American War
In 1885 the Board of Fortifications chaired by Secretary of War William C. Endicott made sweeping recommendations for new coast defenses. Most of these recommendations were adopted in what became known as the Endicott program. Before the Spanish–American War and continuing in the following few years, major construction took place to upgrade the defense capabilities of the three forts defending the major ports along the Delaware River. Fort Mott and Fort DuPont were built essentially from scratch, and a new heavy gun battery was constructed inside Fort Delaware. At all three forts, Endicott-era batteries were built that mounted long-range rifles, mortars, and rapid-fire guns. Construction at Fort DuPont began in 1897, with all but one battery completed by the end of 1900.

Batteries Rodney and Best, with eight  mortars each, were the largest batteries at the fort. All sixteen mortars were named Battery Rodney in 1902, then split in 1906. These were in an "Abbot Quad" battery of four pits with four mortars each, arranged in a square and enclosed and separated by high walls of earth and concrete for maximum protection against enemy fire. This was typical of early Endicott mortar installations and was also intended to concentrate the mortars' fire. However, it was soon found to be too cramped for efficient reloading and later batteries had a linear, open-back arrangement. Batteries Read and Gibson mounted two  guns and two  guns, the former on barbette carriages and the latter on disappearing carriages. These were in an unusual combined battery, with the 12-inch guns of Battery Read on either side of the pair of 8-inch guns of Battery Gibson. Battery Ritchie had two  guns on pedestal mounts. Battery Elder, completed in 1910, had two  guns on pedestal mounts. These small-caliber guns were intended to protect the minefield from minesweepers, and were called "mine defense guns".

Battery Rodney was named for Caesar Rodney, signer of the Declaration of Independence and a major general in the Delaware militia. Battery Best was named for Major Clermont L. Best, an artillery officer in the Spanish–American War who died in 1903. Battery Read was named for George Read, signer of the Declaration of Independence and U.S. Constitution, and U.S. senator. Battery Gibson was named for Colonel James Gibson, killed in the War of 1812 at Fort Erie, Canada. Battery Ritchie was named for Captain John Ritchie, an artillery officer killed in the War of 1812 at Lundy's Lane, Canada. Battery Elder was named for Samuel S. Elder, an artillery officer in the Civil War who died in 1885.

On July 22, 1899, Army General Orders, No. 134, officially designated the "battery at Delaware City" as Fort DuPont, named in honor of Rear Adm. Samuel Francis Du Pont. During this time, according to the Fort DuPont Flashes, the post was garrisoned by soldiers of the 4th U.S. Artillery under the command of Maj. Van Arsdale Andruss. Fort DuPont included the headquarters for the three-fort complex, and had more barracks and administrative buildings than the other two forts. In 1901 the heavy artillery companies garrisoning forts were redesignated as coast artillery companies under the Artillery Corps, and in 1907 they became part of the new U.S. Army Coast Artillery Corps. On completion Forts DuPont, Delaware, and Mott were an artillery district, redesignated in 1913 as the "Coast Defenses of the Delaware".

World War I
The overcrowding situation in the mortar battery was relieved by transferring half the mortars (two mortars per pit) to batteries under construction elsewhere. Four of Battery Best's mortars were transferred to Fort Ruger, Hawaii in 1913, and four of Battery Rodney's mortars were transferred to Fort Rosecrans, San Diego, California in 1918. During World War I, Fort DuPont continued serving the role of coastal defense as well as training post for local draftees and deploying artillery units. In 1915, Batteries Read and Gibson were declared obsolete. Following the American entry into World War I, in 1917 Battery Gibson's 8-inch (203 mm) guns were dismounted for potential use as railway artillery on the Western Front. The carriages were dismounted in 1918 and scrapped in 1922. In 1918 Battery Read's 12-inch (305 mm) guns were transferred to Fort Hamilton, Brooklyn, New York. One carriage was scrapped in 1918, one was sent to Fort Hancock, New Jersey. Battery Ritchie's pair of 5-inch (127 mm) guns were transferred to an "emergency battery" at Fisherman's Island, Virginia in 1917–1918. None of the removed guns were ever returned to the fort. Units such as the 7th Trench Mortar Battalion used Fort DuPont for basic and advanced training before heading to France in October 1918. A two-gun antiaircraft battery with M1917  AA guns was built at the fort in 1918.

Between the wars
Fort Saulsbury near Slaughter Beach, Delaware, built 1917–1920 and accepted for service in 1924, effectively replaced the three forts near Delaware City, though these retained mortars, mines, and some guns through early World War II. Fort Saulsbury had four  guns on long-range barbette carriages and was sited to engage the enemy much further down the estuary than the earlier forts. The Harbor Defenses of the Delaware was one of the most extreme examples of coast defense forts being built further seaward as gun ranges increased. Following World War I, Fort DuPont transitioned to a quartermaster depot and also became an engineer post with the arrival of the First Engineers in May 1922. During this time, Battery E, 7th Coast Artillery was the caretaker detachment for the Coast Artillery Corps facilities at Fort DuPont and the other Delaware River forts. On December 12, 1932, six sets of officers' quarters were floated to Fort DuPont from Fort Mott in Pennsville, N.J. One set of quarters was floated over the year prior. From 1934 until 1936, Fort DuPont and the 1st Engineer Regiment were commanded by Col. Ulysses S. Grant III, grandson of President Ulysses S. Grant. Some sources state the two  guns of Battery Elder were relocated to "Delaware Beach" in 1922 (location unclear), and in 1942 further relocated to Reedy Island to protect a US Navy defensive boom as Battery Liston or Battery Elder II, reportedly leaving service later that year.

World War II

During World War II Fort DuPont served as a mobilization station for deploying units. In 1941, following re-designation, the 1st Engineer Battalion departed for overseas service. At the