As early as 1882 the need for heavy fixed artillery for seacoast defense was noted in Chester A. Arthur's Second Annual Message to Congress where he noted: "I call your attention to the recommendation of the Secretary and the board that authority be given to construct two more cruisers of smaller dimensions and one fleet dispatch vessel, and that appropriations be made for high-power rifled cannon for the torpedo service and for other harbor defenses.

This board recommended a large-scale program of harbor defenses at 29 ports, including guns, mortars, and mine fields.

[3][6] c. 1901 the Coast Artillery took responsibility for the installation and operation of the controlled mine fields from the Corps of Engineers; these were planted to be under observation, remotely detonated electrically, and protected by fixed guns.

Most of the changes recommended by this board were technical; such as adding more searchlights, electrification (lighting, communications, and projectile handling), and more sophisticated optical aiming techniques.

Due to rapid development of the dreadnought battleship type, a new 14-inch (356 mm) gun was introduced in a few locations, including Los Angeles, the Philippines, Hawaii, and Panama.

The rapidity of technological advances and changing techniques increasingly separated coastal defenses (heavy) from field artillery (light).

National Guard coast artillery units were also formed by the states to attempt to bring the CAC up to strength in wartime.

The War Department formed a Board of Review that recommended an increase in strength, which resulted in 105 new CA companies in 1916–17, although these were initially undermanned.

After the American entry into World War I, the Coast Artillery as a whole was ordered brought up to strength, and 71 new companies were organized by July 1917.

[10] In response to the rapid improvements in dreadnought battleships, approximately 14 two-gun batteries of 12-inch guns on a new M1917 long-range barbette carriage began construction in 1917, but none were completed until 1920.

Due to low production and shipping priorities, the Army's railway gun contribution on the Western Front consisted of four CA regiments operating French-made weapons.

[12] The US Navy manufactured and operated five 14"/50 caliber railway guns that were delivered in time to support the final Allied offensives.

Three railway mountings for the Chilean 12-inch guns were ready for shipment by the Armistice; the remaining three barrels were retained as spares.

[21] The official birthday of the Army Warrant Officer Corps is 9 July 1918, when an Act of Congress established the Army Mine Planter Service as part of the Coast Artillery Corps, replacing previous civilian manning of mine planter vessels.

Circular concrete platforms called "Panama mounts" were added to existing defenses to improve the utility of these guns.

Based on the Coast Artillery's experience operating heavy weapons in World War I, especially the French-made 400 mm (15.75-inch) Modèle 1916 railway howitzer, new barbette carriages were designed with an elevation of 65 degrees to allow plunging fire as enemy ships approached.

[28] These were the last guns added to the Philippine defenses until 1940, as the Washington Naval Treaty prohibited additional fortifications in the Pacific.

These units were composed primarily of Filipino enlisted men and US officers, and garrisoned many of the coast defenses in the Philippines until the surrender of US forces there in 1942.

[30] In 1923–1924, the Coast Artillery adopted a regimental system forcewide, which included the Regular Army, National Guard, and Organized Reserve components (see "Units" section below).

The outbreak of war in Europe in September 1939 and the Fall of France in June 1940 greatly accelerated US defense planning and funding.

[25] With war on the horizon, the Navy released the approximately 50 remaining guns, and on 27 July 1940 the Army's Harbor Defense Board recommended the construction of 27 (eventually 38) 16-inch two-gun batteries to protect strategic points along the US coastline, to be casemated against air attack.

[36] Activation of the National Guard and expansion of regular harbor defense regiments to wartime strength resulted in 45,000 troops assigned to this function by fall 1941.

In 1943–44, with most of the new defenses completed, the numerous older weapons of the Endicott and Taft periods were scrapped, with their crews largely reassigned to field artillery units.

Bermuda had been the headquarters and main base of the Royal Navy's North America and West Indies Squadron since the independence of the United States, and the location of its dockyard.

Although Bermuda had been heavily fortified over the previous centuries, and hundreds of artillery pieces had been emplaced, most were hopelessly obsolete.

Eight 8-inch railway guns had been deployed to the Philippines in 1940, but six were destroyed by air attack while entrained in response to the initial landings, and the other two were placed in fixed mountings on Corregidor and Bataan, but lacked crews and ammunition.

Two times a post-1895 military base in the continental United States came under attack were the bombardments of Dutch Harbor, Alaska and Fort Stevens, Oregon by the Imperial Japanese Navy in June 1942.

For the former, members of the 206th Coast Artillery Regiment lost seven during the battle in which the Japanese planes inflicted moderate damage to the base.

In late 1942, the War Department decided that to free up more younger and physically fit troops for frontline duty, harbor defense and anti-aircraft units in the continental United States would be staffed primarily with "limited service" troops, who generally were not permitted to serve on the front lines due to age or disability.

However, many of the Reserve units had only small numbers of, or widely dispersed, personnel assigned, which hampered effective training.