Seacoast Defense in The United States - Between The Wars

Between The Wars

Airplanes were a minor factor in World War I, but the threat prompted changes to coastal defenses in the 1920s and 1930s. Demonstrations in the 1920s by US general Billy Mitchell showed the vulnerability of warships to air attack; this illustrated the use of aircraft for seacoast defense against ships, but also the vulnerability of defenses against air power. In the isolationist US, bombers were seen as more of a defense against naval attack than a strategic offensive weapon. However, planes like the Boeing B-17, which evolved as defensive weapons, turned out to have excellent offensive capacity as well.

In 1922, 274 Coast Artillery companies were authorized and 188 were active. During the year 44 more companies were inactivated, but 14 new companies were created for the Philippine Scouts, and a 15th in 1923. The general staff reconfirmed a commitment to artillery and mines as the most practical and cost-effective methods for seacoast defense, as an alternative to a larger navy or air corps. In 1924 the CAC returned to a regimental system, consolidating the companies into 16 harbor defense regiments and two of Philippine Scouts. The total companies authorized were 289, with 144 active. From 1930 to 1932 the army drafted new defense projects for each harbor. In 1931 it established a Harbor Defense Board to supervise the execution of these projects.

Existing batteries were camouflaged, but if detected, they remained vulnerable to air attack. Therefore, the next, and last, generation of coastal artillery was mounted under thick concrete shields covered with vegetation to make them virtually invisible from above. In anticipation of a conflict with Japan, most of the limited funds available between 1933 and 1938 were spent on the Pacific coast. In 1939 the threat of war in Europe prompted larger appropriations and the resumption of work along the Atlantic coast.

The end of naval armament limitations during the 1930s allowed for larger and longer-range weapons on ships, which reemphasized the need for better long-range shore batteries. Large guns firing 16-inch-diameter (410 mm) 2,000-pound (910 kg) shells with ranges up to 25 miles (40 km), which rivaled the latest naval weapons, were authorized, but there were few of them. Construction for the first 16-inch emplacement began in 1936 at Battery Davis in Ft. Funston, south of the Golden Gate Bridge in San Francisco, California. The first test firing took place in 1940, from Battery Townsley in Fort Cronkhite, north of the Golden Gate Bridge.

In anticipation of war, additional mines, searchlights, radar, and anti-aircraft guns were installed in 1940 and 1941. After the war began, the entire Western Defense Command was placed on high alert, but Japanese attacks, including two submarine attacks and an explosive balloon, caused only minor damage.

Submarine nets and underwater mines protected many harbor entrances. Radar and patrol planes could detect enemy vessels at long distances, and aircraft became the first line of defense against intruders.