The fort was named for Major General Henry W. Slocum, a Union corps commander in the American Civil War.

The study called for better protection of ports such as New York Harbor, and Davids Island became part of its system of defenses.

[1][6] (Despite the presence of mis-captioned postcards, including those in the New Rochelle Public Library, there were never large-caliber breech-loading disappearing guns placed at Davids Island.

[1] With improved dreadnought battleships and the construction of the Coast Defenses of Long Island Sound at the beginning of the 20th century, these batteries became obsolete, and Ft. Slocum was removed in 1907 from the Artillery District of New York (Coast Defenses of Eastern New York from 1913), leaving Fort Totten and Fort Schuyler in that role until 1935.

[6] At the start of the 20th Century Davids Island became the East Coast assembly point for units assigned to America's new overseas territories.

[1] On 16 May 1941, as war raged in Europe, Fort Slocum became part of the New York Port of Embarkation, becoming a staging area for troops moving overseas.

[1] The fort was a key element of the Army's Transportation Corps, so named in mid-1942, whose mission was moving huge numbers of men and amounts of materiel overseas.

Battle-hardened soldiers returning from Europe were put through a "Provisional Training Center" at Fort Slocum to re-acquaint them with the stateside Army, with its surplus of proper military appearance, courtesy, and discipline, along with its deficit of actually shooting Germans.

In May 1944 Private Willie Lee Duckworth of Sandersville, Georgia devised the famous "Sound off, one, two" military cadence while attending one of these classes.

[10][11][1] In November 1944, as the transportation school wound down, Fort Slocum took on a mission of rehabilitating soldiers who had been court-martialed in Europe and sent home.

[1] Following World War II, Fort Slocum was briefly considered as a nuclear research center; what became Brookhaven National Laboratory was chosen instead.

Over the course of this time, troops from the various services, officers and enlisted, male and female, American and allied, were trained in applied journalism, oral communication, radio/TV broadcasting, public and world affairs, and photography.

During the decades that followed, the facilities of the former Army post were neglected and deteriorated severely and continued to occupy Davids Island into the beginning of the 21st century.

[16] The famous "Sound off, one, two" military cadence was invented at Fort Slocum in May 1944, attributed to Private Willie Lee Duckworth of Sandersville, Georgia.