After the United States entry into the Spanish American War, Tampa citizens demanded increased coastal defenses.  In November 1898, construction began on a new military post on Mullet Key.  Fort De Soto, named in 1900, featured 29 post buildings, including barracks, hospital, stable, guardhouse, administration office, mess hall and kitchen, bake house, storehouse, and workshops.  The primary building, Battery Laidley, housed eight 12-inch M 1890 MI mortars with a range of 1.25 to 6.8 miles.  The secondary building,

Battery Bigelow, held two 3-inch, 15-pound Driggs-Seabury Model 1898 rapid-fire guns.

These guns were needed to stop smaller, faster vessels, and to protect the 1.25-mile area surrounding the fort that Battery Laidly could not.  Four mortars from Battery Laidley remain, and are the only mortars of their kind in North America.  The only other ones in the world are in the Philippines.  Battery Bigelow collapsed into the Gulf of Mexico but its ruins can be seen in the surf southwest of Battery Laidley.  The Ft De Soto Batteries and Military Post was listed on the National Parks Register of Historical Places on 1977, and was designated a local historic landmark by Pinellas County in 2014.