12×18 Poster of the Fort Jefferson- Dry Tortugas National Park Cannons
Construction on Fort Jefferson began in 1846 as part of the United States’ effort to strengthen coastal defenses as a result of how easily the British were able to blockade and even invade American cities during the War of 1812. This included successfully burning Washington, D.C., to the ground. Today you might wonder what’s so bad about that, but back in the early 1800s, it started a national panic. This age of fort construction was termed the “Third System,” and from 1816 through 1867, forty-two forts were built. Many of the forts that are still standing today are part of the National Park system.
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.