In 1900, Fort De Soto was garrisoned by a detachment of Company A, 1st Artillery. In 1901, artillery ranks were reorganized and the company became the 1st Company, Coast Artillery. In April 1907, the 1st Company shipped out and was replaced by the 39th Company, Coast Artillery, which stayed at Fort De Soto for three years until June 8, 1910. The number of troops had been cut with the act of February 2, 1901, which created the Coast Artillery. Supplemental troops would be provided by state forces in the event of an invasion.
There were three joint maneuvers at Fort De Soto with the U.S. Army and state troops. In 1907 and 1908, the 1st and 2nd Infantry, Florida State Troops, and in 1909 the 1st Company, Coast Artillery Corps, National Guard of Florida, participated in training exercises.
Eight months after the third maneuver, the Fort De Soto garrison was transferred to Fort Morgan, Alabama, and the fort became inactive with just a caretaker detachment remaining. By September 1914, there was one sergeant and a game warden from the Department of Agriculture remaining on the island. At that time, Mullet Key was being used as a hunting preserve for Fort Dade.
During the first months of 1917, the Army had assigned one non-commissioned officer and eight privates to Fort De Soto. Another group of soldiers arrived to dismount and ship four of the 12-inch mortars to Fort Rosecrans in San Diego, California. Through most of World War I, the detachment had twenty-two privates, two noncommissioned officers, and two officers.
On November 23, 1922, the Secretary of War wrote a letter to the Governor of Florida explaining that the Army would be closing Forts Dade and De Soto. The following are excerpts of that letter:
"Modern developments in armament required considerable modifications in our coastal defense plans and the defense of much of our coastline can now be better accomplished by utilizing mobile artillery instead of fixed armament. The maintenance of Forts Dade and De Soto is not now essential to the coast defense, and sufficient mobile artillery is available to protect Tampa. The limited personnel for Coast Artillery purposes, the question of future appropriations, and the damage wrought by the severe storm in October 1921 were also considerations."
On May 25, 1923, both forts were abandoned, leaving one caretaker at each post.