During World War II, the United States Army underwent significant changes and played a crucial role in the conflict, fundamentally shaping its purpose and structure. The primary objective of the U.S. Army during this period was to mobilize and deploy forces to combat Axis powers, including Germany, Italy, and Japan. This required a vast expansion of personnel and resources, leading to the largest military mobilization in American history up to that point.

In response to the war's demands, the Army implemented several reforms. The establishment of the Selective Service Act in 1941 allowed for the conscription of millions of American men, dramatically increasing the size of the Army from about 190,000 active-duty personnel in 1940 to over 8 million by 1945. Training programs were enhanced, and new units were formed, including specialized divisions like the Army Air Forces, which emphasized the importance of air power against the Axis powers of Adolf Hitler, Benito Mussolini and Hideki Tojo.

The Army also adopted more modern organizational structures and tactics, reflecting lessons learned from earlier conflicts. Innovations in strategy, logistics, and technology were prioritized to improve combat effectiveness. The integration of women into the military through organizations like the WAVES (Women Accepted for Volunteer Emergency Service) and the WACs (Women's Army Corps) marked a significant shift in personnel policy, although racial segregation remained a prominent issue within the forces.

Command, control and organization At the beginning of the war, the Army and Air Force were under the command of the War Department, which in turn was subordinate to the President. The command structures were the same as those used during World War I. In 1940, there were six branches within the Army that were considered combat arms: Infantry, Cavalry, Field Artillery, Coast Artillery, Air Corps and the Corps of Engineers.

The War Department had authorized a General Headquarters, United States Army (GHQ) in 1921, but it was not activated until July 26, 1940. Its purpose was to facilitate mobilization by overseeing the organization and training of the Army's field forces on the American mainland (Zone of Interior). In October 1940, the command of the corps areas was separated from the four armies that had been established in 1932. On March 17, 1941, the United States was divided into four defense commands that would become operational areas in the event of an impending invasion. In addition to these training duties, GHQ was assigned command and planning for military operations in the Zone of Interior on July 3, 1941. GHQ was replaced by the Army Ground Forces on March 9, 1942. The four defense commands and all operational areas were transferred to the War Department General Staff.

General Staff

The US Army General Staff was the centerpiece of strategic planning and coordination. It consisted of several departments: G-1 (Personnel): Responsible for personnel management, recruiting and troop strength. G-2 (Intelligence): Responsible for gathering and analyzing intelligence on the enemy. G-3 (Operations): Coordinated the planning and execution of military operations. G-4 (Logistics): Focused on supplying troops with supplies and equipment.

The head of the US Army was the Chief of the General Staff, a role filled by General George C. Marshall during World War II. There was also a special staff consisting of the Legislative and Liaison Division, the Inspector General, the Manpower Board, the Budget Division and the Civil Affairs Division.

Field army A field army theoretically consisted of three corps, an anti-aircraft artillery brigade, engineers with three general regiments, six separate battalions, two heavy pontoon battalions and a battalion for topography, water supply and camouflage, three tank destroyer battalions, two signal battalions and a large number of companies and service units. A total of 11 armies were formed during the Second World War.

First United States Army (October 1, 1933) Second United States Army (October 1, 1933) Third United States Army (October 1, 1933) Fourth United States Army (October 1, 1933) Fifth United States Army (January 5, 1943) Sixth United States Army (January 25, 1943) Seventh United States Army (July 10, 1943) Eighth United States Army (June 10, 1944) Ninth United States Army (April 15, 1944) Tenth United States Army (June 20, 1944) Fifteenth United States Army (August 21, 1944)

The First, Third, Fifth, Seventh, Ninth and Fifteenth Armies served in Europe, while the Sixth, Eighth and Tenth Armies were deployed in the Pacific. The Fifth and Seventh were deployed to North Africa, while the Second and Fourth remained in the United States and took on training duties.

Corps The corps theoretically consisted of three infantry divisions, an anti-aircraft regiment, a mechanized cavalry regiment, two engineer regiments, a field artillery brigade, a tank destroyer group, a signal battalion and various service units. The size of corps staffs was drastically reduced by Army Ground Forces in 1943 as the corps concept evolved along flexible guidelines that allowed for individual combat battalions to be assigned or attached as needed. A corps became a variable combination of divisions and battalions, essentially consisting of a commander and a small staff directing combat operations and leading the non-divisional combat units under his command. These included elements such as the corps artillery, cavalry squadrons and tank destroyer battalions. They were distributed among the divisions, deployed for massive support or held in reserve. As the divisions had been streamlined, the corps became the headquarters for the deployment of combat elements in the right tactical combinations for the task at hand.

Division Divisions were the largest units of the army with a prescribed structure and functioned as the smallest formation comprising units of different arms and services. They were designed for normal operational self-sufficiency, which varied depending on the type of division. Since it was assumed that the armored division would normally operate at a considerable distance from the mass of forces, as would the mechanized division at a somewhat lesser distance and the infantry division the least, maintenance, supply, and engineering support was distributed accordingly. All were kept to a minimum, as corps and armies were designed as pools of specialized units to provide needed support in unusual circumstances.

During the Second World War, a total of 92 divisions were deployed, 89 of which remained after the war. By May 1945, the five hardest hit divisions (3rd, 4th, 9th, 36th and 45th) had suffered a loss of 176%. To partially offset these losses, the divisions not yet deployed were ruthlessly plundered. This wiped out much of the careful preparation and training programs that had prepared the divisions, and many of them were deployed as hastily upgraded formations with minimal training. A typical infantry division consisted of 3 infantry regiments, a reconnaissance platoon, four artillery battalions, an engineer battalion, a medical battalion and six special units such as a signal company, military police and military band.

The initial armored division consisted of a tank brigade with three tank regiments, two with light tanks and one with medium tanks, as well as a field artillery regiment with two battalions. There was also an armored reconnaissance battalion and an attached air observation squadron for reconnaissance, while the support and service elements consisted of an armored infantry regiment, a field artillery battalion, an engineer battalion, a signal company, a maintenance company, a QM truck battalion and a medical battalion. With the second major reorganization, which came into effect on September 15, 1943, the light tank division was introduced. It reduced the tank strength by replacing the two regiments (i.e. six tank battalions) with three tank battalions. The regimental level was abolished in all armored divisions with the exception of the 2nd and 3rd, which remained in the old structure. This should have meant a halving of the armored division (three instead of six battalions); in fact, the reduction amounted to about a third. An additional combat command was introduced to control the division reserves, which was designated CCR (or CCC). The reconnaissance battalion was replaced by a cavalry reconnaissance squadron, reducing the total strength of the division by almost 4,000 men. During the Second World War, the armored divisions underwent six separate reorganizations, two of which were actually significant. The first, which came into effect on March 1, 1942, resulted in heavy tank divisions. The tank brigade was disbanded together with one of the tank regiments, so that two combat commands (CCA and CCB) and two tank regiments were created in its place. There were three tank battalions in each regiment, whereby the ratio of light to medium tanks was changed and each regiment now had two medium and one light battalion. The artillery was also reorganized into three separate battalions. The combat commands gave the division more flexibility, as their composition could be varied according to the division commander's assessment of the battle situation.

Regiment, company and platoon The strength of the Standard Infantry Regiment changed several times during the course of the war. In June 1941, it was increased from 2,542 to 3,340 men and then reduced to 3,118 men by July 15, 1943, and to 3,068 men by January 24, 1945. An infantry regiment consisted of a staff company, three infantry battalions, an anti-tank company, a gun company and a support and supply company. An infantry battalion consisted of 871 men in four companies plus staff. The companies were 187 men strong and consisted of four platoons. A company was commanded by a captain, a platoon by a lieutenant or sergeant. At full strength, a platoon had 3 squads of 12 men.

Recruitment

At the start of the war in December 1941, the army had a strength of 1,686,000 soldiers divided into 29 infantry, 5 armored and 2 cavalry divisions. Of these divisions, only two were outside the continental United States, the remaining 34 lacked essential equipment and only 17 had received sufficient training to be considered combat ready. By 1945, the number of troops had grown to 7.7 million soldiers. The army consisted of the Regular Army, the National Guard and the Reserve. The Selective Training and Service Act of September 16, 1940, set the draft age at 21 to 45. Recruitment was carried out by the Selective Service. The Selective Service consisted of a national office and 53 branch offices spread across the individual states. In addition to an office for each of the 48 states, there was also an office in Hawaii, Alaska and Puerto Rico as well as one each for the city of New York and the District of Columbia.

At the local level, physicians appointed to 6,443 local boards were responsible for initially assessing the physical condition of registrants. In contrast to the process employed during World War I, the Selective Service System did not induct men into the service. The final decision regarding a man's qualifications for service was not made by the local board, except in cases where the man was rejected outright. The local board considered all aspects of civilian manpower requirements in order to determine whether a man was available for service.