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Antiaircraft artillery was descended from the Coast Artillery Corps, and did not gain a separate identity until 1943. Antiaircraft units, like the tank destroyer, had been massively expanded in the wake of the German blitzkriegs of 1939 and 1940. Hundreds of battalions were formed in 1940-43, but many were made redundant by the almost complete Allied air superiority of 1944. Many battalions were disbanded in order to provide replacements for the infantry. However, every division had a antiaircraft battalion attached. These towed battalions consisted of four batteries equipped with a total of sixty-four weapons, equally split between the M-1 40mm Bofors gun and the M-55 quad .50-caliber machine gun mount. The armored divisions had a self-propelled antiaircraft battalion attached. Like its towed counterpart, the SP battalion had sixty-four pieces divided into four batteries. Half of these were the M-15 Machine Gun Motor Carriage which consisted of a 37mm gun and two .50-caliber machine guns. The other thirty two mounts were the M-16 MGMC, each carrying a M-55 mount fitted to a halftrack. Heavy antiaircraft support was provided by the gun battalions, which deployed sixteen 90mm guns in four batteries. The 90mm gun was an accurate, high-velocity piece, full capable of engaging ground targets. It was similar in performance to the famous German FLAK 88, and, in some ways, was its superior (its data transmission system, automatic fuse-setter, and high-speed automatic rammer were second to none in design and performance). Enhancing the lethality of the 90mm was the proximity fuse, which had originally been designed for antiaircraft use as well as the use of portable early-warning and fire-control radar sets.

Towed battalions assigned to divisions were all designated as mobile, which meant that there was a full complement of prime movers for the guns. Some antiaircraft battalions were designed as semi-mobile, which meant that there was only one prime mover per two or three guns. Semi-mobile battalions were intended for the static defense of installations.

Automatic weapons battalions were normally assigned directly to a division or were part of an antiaircraft artillery group, which consisted of a headquarters and headquarters battery and two or more attached battalions. Groups assigned to a corps usually consisted entirely of automatic weapons battalions, but occasionally had one or more gun battalions. Most gun battalions assigned to groups were held directly under an army headquarters. Separate from army control were the antiaircraft battalions of the IX Air Defense Command. This had originally been part of the Ninth Air Force but had been detached in August 1944 and placed directly under SHAEF control. The IX Air Defense Command was tasked with the security of the various air bases and other rear-area installations scattered throughout Belgium, France and Luxembourg. Most of the semi-mobile battalions in the ETO were assigned to the IX Air Defense Command.

The increasing threat of the German V-1 weapons to the port of Antwerp in the fall of 1944 gave the IX Air Defense Command a new mission. The Allies deployed large numbers of antiaircraft artillery units in a corridor (soon known as buzz-bomb ally) that extended north from the town of Elsenborn, Belgium almost to the sea. This massive concentration of guns was to prove a fortuitous and welcome addition to the Allied defense in the early days of the Battle of the Bugle.