The first army antiaircraft units had been formed on 10 October 1917. By September 1939 the large proportion of Coast Artillery available was antiaircraft in nature, and as the threat of enemy invasion faded, coast artillery personnel and assets were increasingly transformed into Antiaircraft Artillery units. By the end of the war the seacoast defense role and, consequently, Coast Artillery had practically disappeared, and Antiaircraft Artillery prevailed. The World War II mission of Antiaircraft Artillery was the air defense of field forces and ground installations against all forms of enemy air attack by day or night.

While Coast Artillery barrage balloon, automatic weapons, antiaircraft gun, and searchlight battalions were being phased into the new Antiaircraft Artillery, one type of Coast Artillery battalion remained viable. This was the handful of 155mm Long Tom gun battalions used throughout World War II in the Pacific. These were mobile extensions of harbor defense artillery, but used as normal heavy artillery during the numerous island campaigns in which it participated. Coast Artillery still retained fixed fortifications artillery in numerous harbor forts, although by the end of World War II these were mostly in a maintenance or "caretaker" mode. At the close of World War II the Coast Artillery also had nine mine-planter batteries and three junior mine-planter batteries based aboard Type 1, 2, and 3 army mine-planter cable vessels.

The Antiaircraft Command was established 9 March 1942 at Washington, DC with the mission of instructing and training personnel for duty with antiaircraft artillery and barrage balloon units and organizing and training such units for combat duty. The headquarters was moved to Richmond, Virginia, on 23 March 1942, and to Fort Bliss on 13 October 1944, and was discontinued there 30 October 1945.

In the three years following 31 December 1940 antiaircraft artillery increased over 1,750 percent, with a 2,400 percent increase projected by the 811 battalions the Army Ground Forces requested on 30 September 1942. Thereafter, Army Ground Forces repeatedly advised reduction, believing that provision for the Army Air Forces was sufficient to gain aerial supremacy, enabling antiaircraft strength to be placed into units of higher combat value as a result. The War Department hesitated to curtail the antiaircraft program until the Troop Basis of 4 October 1943, when the planned figure was reduced to 475 battalions. Even after this reduction, antiaircraft artillery units active at the end of 1943 had an authorized strength nearly four times that of nondivisional field artillery. About 100 battalions were inactivated, until the total fell to 460 in 1944. By 1 April 1945, 331 antiaircraft artillery battalions of all types were in existence.

No other ground arm had to ship its units into combat as rapidly due to the heavy demand for antiaircraft protection early in World War II. This requirement extended from overseas bases to defense installations within the United States, and as a result units were shipped out with less than 12 weeks' training. Although poorly trained, they still took the best personnel and equipment, which not only harmed the training and cadre base, but led to undisciplined firing on friendly aircraft. The latter problem was only corrected by ordering withholding of fire in certain zones even when attacked by enemy airplanes, and the former problem largely corrected itself. By the end of 1943 equipment for training was more plentiful, the supply of antiaircraft units was coming into a more favorable ratio to overseas demand, and the number of new units to be trained declined. At the same time antiaircraft functions steadily increased and became more complex. For example, the role of antiaircraft artillery in a supplementary ground support role became a major doctrinal practice. As the antiaircraft artillery program was checked, and then slashed, other units such as infantry made up personnel shortages and large numbers of replacements were made available for overseas duty with depleted divisions. Many of the antiaircraft troops whose training caused such concern ended up in the infantry, and some regular antiaircraft units were also utilized in this capacity as well as with the diminished enemy air threat in certain theaters.

Initially the Coast Artillery (Antiaircraft) Brigade was a fixed organization which contained 7,015 personnel. On 24 December 1942 permission was granted to reorganize antiaircraft units under the flexible army group/battalion system which abolished the old brigade structure and implemented tactical headquarters at brigade and group level to which subordinated battalions could be freely attached. Due to the large quantities of antiaircraft artillery the brigade headquarters became a widely used organization, usually assigned to army level and each controlling three like groups as well as several directly attached battalions.

Coast Artillery Brigades (Antiaircraft) were first activated or inducted into federal service during January-February 1941, and most were redesignated as Antiaircraft Artillery Brigades on 1 September 1943. These brigades declined in number as antiaircraft battalions were inactivated, and from October 1944 in Europe they normally controlled only two groups and a reduced number of independent battalions attached directly to brigade level.

Beginning in August 1942, Coast Artillery Groups (Antiaircraft) were raised in quantity, and these were redesignated as Antiaircraft Artillery Groups during May-June 1943. These groups were primarily redesignations of former Coast Artillery regiments being broken up under the guidelines of the group/battalion system. This continued through 1943 and into 1944. Additionally, the first Barrage Balloon Group was activated on 1 February 1942 and another followed on 1 May 1943, both being inactivated in September 1943.

Several types of Coast Artillery Groups were formed without antiaircraft roles. In August 1944 several Coast Artillery Groups (Harbor Defense) were redesignated Coast Artillery Regiments in Hawaii, and in November 1944 another such conversion was made in Panama. Two Coast Artillery Training Groups, the 17th and the 18th, existed at Camp Davis (North Carolina) from 10 March 1942 to 15 May 1942. Seven Coast Artillery Groups (155mm Gun) were activated, and three saw combat in the Pacific at New Guinea, Luzon, and Okinawa. The others were converted into field artillery groups or disbanded.

By far, the most numerous were the Coast Artillery Regiments (Antiaircraft). These establishments averaged 2,304 if mobile and 2,155 if semimobile. Often, their mobility status was freely altered depending on operational requirements, and the designation as one particular type in the written record should not be taken as permanent. Many existed in the prewar Regular Army and more were activated through 1942, the National Guard regiments of this type being inducted beginning in September 1940. All were broken up in 1943; normally, the regimental headquarters being redesignated as an antiaircraft artillery group headquarters, its 1st Battalion becoming a separate Antiaircraft Artillery Gun Battalion, the 2nd Battalion becoming a separate Antiaircraft Artillery Automatic Weapons Battalion, and the 3rd Battalion becoming a separate Searchlight Battalion.

The Coast Artillery also had one 8-inch railway gun regiment of 2,040 men, a prewar organization broken up on 1 May 1943. Several 155mm Gun regiments (each 1,754 men) were raised or inducted commencing in 1940, and were broken up January-June 1944, with their battalions separated as independently numbered units. The Coast Artillery had numerous Harbor Defense regiments, most of them being of prewar vintage in reduced status at harbor forts. In September 1940, National Guard units of this type were inducted and positioned in harbor defenses of their home states. During March-October 1944 these regiments were either absorbed into their harbor defenses served, or broken up through inactivation and their battalions renumbered as separate entities. The Type C fixed harbor defense regiment contained 2,502 personnel; the Type A contained 1,943 personnel; the Type B contained 1,388 personnel; and the Type D contained only 655 personnel.

16-inch gun in concrete casemate, Fort MacArthur, California.

Harbor defenses existed or were established at virtually every harbor facility in the United States and its territories. to include the Panama Canal Zone. These were organizations highly tailored to the specific conditions of defense necessary in each case, and were usually manned under individual tables of distribution and allowances. Harbor defenses scattered their assets among nearby or controlling forts, camps, gun emplacements and positions, seachlight points, outposts, subposts, reservations, tactical positions, and battery sites.

6-inch gun in a shielded barbette mount, Battery 247, Fort Columbia, Washington.

A wide variety of coast or antiaircraft artillery battalion types existed during World War II. These were equipped with 37mm M1A2 AA guns, multiple-mounted .50-caliber machine guns, twin 40mm gun motor carriages M19, Bofors 40mm automatic AA guns M1, 3-inch AA guns M3, 90mm AA guns M1 and M1A1, and 120mm AA guns M1. Battalions included a number of specialized types: Harbor Defense (with variable components), Composite (combined antiaircraft/seacoast weapons), 155mm Long Tom Gun, Antiaircraft Artillery Automatic Weapons, Antiaircraft Artillery Gun, Railway 8-inch Gun, Barrage Balloon (including low-altitude and very-low-altitude variants), Airborne Antiaircraft Artillery (with flexible machine gun battery attachment, or fixed combination of automatic weapons and machine gun complements for airborne division use), Searchlight, Antiaircraft Artillery Machine Gun, and Seacoast Training battalions.

Battalion mobility status in this document reflects that of the unit during the majority of its combat operations, or majority of service if not deployed overseas into a combat theater. Once battalions departed the United States, their mobility designations rarely changed, even though many were stripped in theater by divisions and higher commands for their trucks and other vehicles.

The Antiaircraft Artillery School was activated at Camp Davis on 31 March 1942 and moved to Fort Bliss in October 1944, where the headquarters of the Antiaircraft Artillery School was already located. Antiaircraft artillery equipment was initially tested and developed at the Coast Artillery Board at Fort Monoe. On 9 March 1942, a separate Antiaircraft Artillery Board was established there and moved to Camp Davis on 24 May 1942. Finally, on 28 August 1944, the board moved to Fort Bliss to join what became the center of army antiaircraft activities. The Coast Artillery Board had existed since 1907 at Fort Monroe and was charged with review and development of harbor defense weapons, which included mine planters, underwater detection devices, submarine mines and mine-control devices, and , prior to March 1942, antiaircraft weapons.

Three Coast Artillery Replacement Centers began operation in March 1941. In March 1942 these were separated into antiaircraft artillery and seacoast establishments. The former were located at Fort Eustis (later Camp Stewart) and Camp Callan (later at Fort Bliss). The Camp McQuade, California, Coast Artillery Replacement Training Center – handling the seacoast establishment function – was activated 12 July 1942 under the Replacement and School Command and operated until December 1943.

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