army, U.S. Department of Defensea large organized force armed and trained for war, especially on land. The term may be applied to a large unit organized for independent action, or it may be applied to a nation’s or ruler’s complete military organization for land warfare.
Throughout history, the character and organization of armies have changed. Social and political aspects of nations at different periods resulted in revision in the makeup of armies. New weapons influenced the nature of warfare and the organization of armies. At various times armies have been built around infantry soldiers or mounted warriors or men in machines. They have been made up of professionals or amateurs, of mercenaries fighting for pay or for plunder, or of patriots fighting for a cause. Consideration of the development of armies must be made in the light of the times in which the particular army was forged and the campaigns that it fought. (See also war.)