Antiaircraft Artillery

At the beginning of World War II the US antiaircraft artillery force was very much the poor stepchild of the Coast Artillery Corps. The units were mostly three battalion (a gun battalion, an automatic weapons battalion, and a searchlight battalion) regiments and separate battalions. They were equipped with a motley mix of obsolescent 3" guns and single barrel water-cooled .50 caliber machine guns. The German Blitzkrieg in Europe forced a widespread reevaluation of the Army's AAA capability and, beginning in 1940-1941 a vast expansion of the arm (it finally achieved an identity separate from the Coast Artillery in 1943). On 30 September 1942, it was proposed that 811 AAA battalions be organized (with a total strength of 619,000 men).

However, this massive buildup of AAA units became largely redundant when another formerly poor relation of the US Army, the Army Air Corps, wrested command of the air from the Luftwaffe in 1943 and 1944. Many AAA battalions were disbanded to provide replacements in 1944, some were converted to artillery. A total of 258 battalions were inactivated or disbanded between 1 January 1944 and 8 May 1945. Nevertheless, AAA remained a strong component of the army and achieved something of resurgence in late 1944 in Belgium, defending Antwerp from the threat of the V-1 "Buzzbomb." On 31 December 1944, there were still a total of 347 AAA battalions (with 257,000 men) active in the Army.

In 1943 the AAA regiments were broken up into separate battalions, with the regimental HQ & HQ companies becoming new AAA Group Headquarters. The AAA battalions were organized as either gun (equipped with the M1 90mm AA gun) or automatic weapons (equipped initially with a U.S.-designed M1 37mm gun, but later almost wholly re-equipped with the famous M1 40mm Bofors-designed gun, and with the M51 or M55 quad-mount .50 caliber machine gun). Battalions were further classified as mobile (that is towed), SP (utilizing half-track-mounted guns, the M16, a quad .50 caliber mounting, and the M15, a combination mounting twin water-cooled .50 caliber and a single 37mm), or semi-mobile (with a reduced number of prime movers, designed for the defense of static installations).

The automatic weapons battalions of all types were organized with four firing batteries lettered A to D, an HQ & HQ Battery, and a Service Battery. Each battery nominally contained eight towed 40mm or 37mm SP guns and eight quad .50 caliber towed or SP machine guns. However, many slight variations existed, some battalions had batteries composed of eight towed 40mm guns, four quad .50 caliber mounts, and eight single, water-cooled .50 caliber machine guns (there was a shortfall in production of the M51 and M55 mounts). Gun battalions were organized identically, except the batteries were equipped with four 90mm guns each and, usually, three water-cooled .50 caliber machine guns.

Normally, an AAA automatic weapons battalion was attached to each division, SP if attached to an armored division, and mobile if attached to an infantry division. A corps normally had one or more AAA groups attached. Each AAA group consisted of two or more automatic weapons battalions (usually mobile), although a gun battalion was occasionally attached. In the European Theater, gun battalions were more frequently found in AAA groups attached to the army or army group. Antiaircraft brigades were also formed and were normally attached to armies or to theater commands. In addition, the IX Air Defense Command (in effect an AAA division, originally organized as a part of the US Ninth Tactical Air Force), with an average of ten to fifteen gun and automatic weapons battalions, formed a powerful AAA reserve for the US 12th Army Group in Europe.