Although armies existed in ancient Egypt, China, India, and Assyria, Greece was the first country known for a disciplined military land force. The Greeks made military service obligatory for citizens and training was rigorous. As a result of Greek military successes, leaders of other nations sought the services of Greek mercenaries. In time, a class of professional soldiers developed. They sold their services to other rulers as well as to wealthy Greeks who chose to avoid required military service (see Xenophon).
Like the Greek armies, the Roman army was originally composed of citizen soldiers. As the Roman Empire expanded, a professional standing army came into being; it became increasingly composed of barbarian mercenaries. The Roman army was divided into legions, each of which included heavy and light infantry, cavalry, and a siege train. The army became a political force that often determined who ruled the empire.
In Islam, slave soldiers were often trained from youth to be loyal only to their owners. These slave armies often established dynasties of their own (see Mamluks; Janissaries). In medieval Japan and Europe, samurai and knights, respectively, owed military service to a lord. The European system depended on the feudal levy, which required knights and yeomanry to provide a fixed number of days of military service per year to a great lord. Because of this limitation on service and the poorly trained force that it produced, sustained military operations were difficult. Feudal armies were undermined by the development in England of the longbow, but they were destroyed by the introduction of gunpowder. Armed knights became easy victims of hand-carried firearms and castle walls could now be breasted by cannon.
National armies, largely composed of mercenaries, reappeared after the introduction of gunpowder. An example is the Italian condottiere, who hired mercenaries to fight for the prince who was able to pay the most. German and Swiss mercenaries served all over Europe in the 15th and 16th cent. Professional soldiers were also a notable feature of the armies of the Ottoman Turks, who threatened to destroy the forces of Western Europe in the 16th cent. Eventually, as a result of the writings of such political theorists as Niccolo Machiavelli, national or standing armies developed—armies of professional soldiers led mostly by officers from the country's aristocracy.
After the Thirty Years War (1618-48), France emerged as the preeminent European military power. Under Louis XIV and his war minister, the marquis de Louvois, that country organized a national standing army that became the pattern for all Europe until the French Revolution. A professional body, set apart from civilian life and ruled under an iron discipline, the standing army reached harsh perfection under Frederick II of Prussia.
In the late 18th cent. the American and French revolutions brought about the return of the nonprofessional, citizen army. The introduction of conscription during the French Revolutionary Wars led to mass armies built around a professional nucleus. Officers could be from any class. Conscription also transformed non-European armies, such as that of Egypt during the early 19th cent.
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.
See A. Vagts, A History of Militarism (1937); L. L. Gordon, Military Origins (1971); J. Keegan and R. Holmes, Soldiers (1986); R. O'Connell, Of Arms and Men (1989).
An army (from Latin Armata "act of arming" via Old French armée), in the broadest sense, is the land-based armed forces of a nation. It may also include other branches of the military such as an air force. Within a national military force, the word Army may also mean a field army, which is an operational formation, usually made up of one or more corps.
In several countries the army is officially called the land army to differentiate it from an air force called the air army, notably France. In such countries, the word "army" on its own retains its connotation of a land force in common usage. The current largest army in the world by number of active troops is the People's Liberation Army of China with 2,250,000 active troops and 800,000 reserve personnel.
A field army is composed of a headquarters, army troops, a variable number of corps, and a variable number of divisions. A battle is influenced at the Field Army level by transferring divisions and reinforcements from one corps to another to increase the pressure on the enemy at a critical point. Field armies are controlled by a General or Lieutenant General.
A particular army can be named or numbered to distinguish it from military land forces in general. For example, the First United States Army and the Army of Northern Virginia. In the British Army it is normal to spell out the ordinal number of an army (e.g. First Army), whereas lower formations use figures (e.g. 1st Division).
In the Soviet Red Army and the Soviet Air Force, "Armies" were actually corps-sized formations, subordinate to an Army Group-sized "front" in wartime. In peacetime, a Soviet army was usually subordinate to a military district.