With the advent of railroads and, later, highway systems it became possible after the mid-19th cent. to move large concentrations of troops, and the nations of the world were able to benefit from enlarging their manpower bases by conscription. Armies changed technologically as well. Trench warfare resulted from improvements in small arms and prompted the development of various weapons designed to end the stalemates and murderous battles that entrenched forces produced. The growing role of artillery made logistics even more important. From the first, armies had needed soldiers to supply the fighting troops—even when the armies simply lived off the land. No formal distinction orginally was made between service troops and combat troops, but with the creation of the great citizen armies after the French Revolution formal specialization proliferated, and quartermasters, ordnance troops, engineers, and medical specialists were organized into separate units. The development of mechanized warfare in the 20th cent. made armies powerful and highly mobile and yet did not always provide them with the capabilities needed to fight so-called asymmetric opponents, such as they face in guerrilla warfare and terrorism.
The term army is still applied to all the armed land forces of a nation, but it is also used to designate a self-contained unit with its own service and supply personnel. In many armies today the division (usually about 15,000 men and women) is the smallest self-contained unit (having its own service and supply personnel). Two or more divisions generally form a corps; and an army (c.100,000 men or more) is two or more corps. In World War II, army groups were created, including several armies (sometimes from different allied forces). Above the groups is the command of a theater of operations, which in the United States is under the control of the Joint Chiefs of Staff.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.