This strategic position afforded it a commanding view over the Atlantic entrance to the commercially vital Long Island Sound.

Despite the new civilian control, the biological warfare mission continued until 1969, when the US officially ended offensive BW research.

[1] First “owned” by the Corchaug and Montaukett Indian tribes[2] the Plum Island was "sold" to Samuel Wyllys for a coat, a barrel of biscuits and 100 fishhooks.

[3] The original fort was constructed after the federal government acquired Plum Island from Abraham S. Hewitt, a former mayor of New London, Connecticut,[4] for $25,000.

A number of weapons were purchased from the UK and hastily mounted to give the nascent fort system some modern armament.

A pair of emplacements for modern 8-inch M1888 guns on modified 1870s-era Rodman carriages were also built at nearby Fort Tyler, but these seem to have not been armed.

As an artillery post, Fort Terry was heavily armed as part of the Coast Defenses of Long Island Sound.

[18] Fort Terry served as an artillery post during the Spanish–American War, and it was intended to attack enemy ships as they headed toward New York City.

[21] Following the American entry into World War I in April 1917, changes were made at the stateside forts with a view to putting some coast artillery weapons into the fight on the Western Front.

[12] In 1924, Fort Terry was used by the Portsmouth (NH) National Guard Armory (the 197th Coast Artillery Regiment (Antiaircraft))[24] as their summer encampment and training location.

[27][28] On 7 December 1941, the fort was listed as manned by the 242nd Coast Artillery Regiment (Harbor Defense) of the Connecticut Army National Guard.

[29][30] Today, on the east side of Plum Island, a network of trenches remains from the area's tenure as an artillery post.

It was small and focused primarily on anti-animal biological warfare (BW) research aimed at enemy livestock.

[17] The lab was not completed by the time the Chemical Corps transferred the fort to the USDA but it and the rest of the remodeled buildings were eventually incorporated into the civilian facility.

[31] Most of the disease research done by the USDA focused on biological warfare until Richard Nixon ended the U.S. bio-weapons program in 1969.

[6] Most of the original buildings and batteries still stand today[7] and in many cases have been incorporated in one way or another into the island's new role as a disease research center.

[35] A 2008 DHS report recommended that the remnants of Fort Terry, its buildings and batteries, be opened to the public and preserved.

[36] The Town of Southold, New York formed a Local Waterfront Revitalization Program (LWRP) which noted that many of the island's structures, including those at Fort Terry, could qualify for listing on the U.S. National Register of Historic Places.

The number of pathogens studied was reduced to two, rinderpest and FMD, and the mission was changed to "defensive" research of those two diseases.