Training division

This list of United States Army divisions is divided into three eras: 1911–1917, 1917–1941, and 1941–present. These eras represent the major evolutions of army division structure (there have been several minor changes during these times). The 1911–1917 era lists divisions raised during the Army's first attempts at modernizing the division, prior to the authorization of permanent divisions, and the 1917–1941 era lists the first permanent divisions, prior to advent of specialized (armored, airborne, etc.) divisions. The 1941–present era lists all of the divisions organized, raised, or authorized since then.

As much as possible, divisions are only listed in the eras in which they were first created. Some divisions, such as the 1st Cavalry Division, are listed in multiple eras, as their organizations were drastically changed from one era to the next. Many divisions overlap the years listed in the era categories, mainly due to the slow pace in which they were deactivated, inactivated, or otherwise disbanded.

It should also be noted that several divisions have existed under multiple designations, such as the 10th Mountain Division (10th Light Division (Alpine), 10th Infantry Division). Additionally, several divisions with the same numerical designations were completely separate and unrelated divisions (there have been two 5th Divisions, for instance).

Divisions in the United States Army have existed since the American Revolution when, on 22 July 1775, George Washington organized three divisions in Boston, Massachusetts. Early American divisions, up until the American Civil War, were primarily temporary organizations, with the basis of the United States Army being brigades and regiments.

During the Civil War, the war in which formed the first large true armies in United States history, divisions were formed primarily to support Army corps, and were usually numbered as 1st, 2nd, and 3rd Division of the pertaining corps.

The concept of the permanent United States Army division was formulated and put to the test following the turn of the 20th century. In 1916, the permanent division would finally be authorized by Congress, resulting in a dramatic change in the Army's force structure. For the first time, the division was the base element of the United States Army and remained as such until the Global War on Terrorism, when the Army switched its emphasis to brigade and regimental combat teams.

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