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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.
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).