High Quality Content by WIKIPEDIA articles! Seacoast defense was a major concern for the United States from its independence until World War II. Before airplanes, America`s enemies could only reach her from the sea, making coastal forts an economical alternative to standing armies or a large navy. After the 1940s it was recognized that fixed fortifications were obsolete and ineffective against aircraft and missiles. However, in prior eras foreign fleets were a realistic threat, and substantial fortifications were built at key locations, especially protecting major harbors.