4×6 Postcard of Dry Tortugas National Park, Artwork of Cannon
In a time before airplanes, any foreign enemy that wished to invade a country on another continent had to do so by ship, and ships could not attack or launch an amphibious assault from just anywhere along the coast. Water deep enough to accommodate a large ship was needed, and a ship had to get close enough to shore so its cannon fire could pound the area and its soldiers could land in their small boats. Of course, most such places are occupied by major cities, for deep water was coveted in peaceful times for its impact on commerce. Thus, in the United States, most of the new forts were built near cities, with one notable exception being Fort Jefferson at Dry Tortugas.