[[image:Conscription map of the world.svg|thumb|300px|Map of the world of the army forms legend: No armed forces legend: No conscription legend: Plan to abolish conscription within 3 years legend: Conscription legend: No information]] 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 lander 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 standing army is an army composed of full-time career soldiers who 'stand over', in other words, who do not disband during times of peace. They differ from army reserves who are activated only during such times as war or natural disasters.

By definition, irregular military is understood in contrast to regular armies which grew slowly from personal bodyguards or elite militia.

Sparta

The Spartan Army was one of the earliest profession armies, as men began training at the age of 7 and devoted their lives to war until retirement at the age of 60. Unlike other civilizations, whose army had to disband during planting and harvest season, the Spartan serfs, or helots did the manual labour. This allowed the Spartans to field a full-time army with a campaign season that lasted all year. The Spartan army was largely composed of hoplites, equipped with arms and armour nearly identical to each other, each bearing the Spartan emblem and the colour scarlet.

Roman Empire

main: Military History of Rome The Roman army was not the first professional army, using the best factors of the Spartan military. It had its origins in the citizen army of the Republic, which was staffed by citizens serving mandatory duty for Rome. The reforms of Marius around 100 BC turned the army into a professional structure, still largely filled by citizens, but citizens who served continuously for 25 years before being discharged.

The Romans were also noted for making use of auxiliary troops, non-Romans who served with the legions and filled roles that the traditional Roman military could not fill effectively, such as light skirmish troops and heavy cavalry. Later in the Empire, these auxiliary troops, along with foreign mercenaries, became the core of the Roman military. By the late Empire, tribes such as the Visigoths were bribed to serve as mercenaries.

Medieval

In the earliest Middle Ages it was the obligation of every noble to respond to the call to battle with his own equipment, archers, and infantry. This decentralized system was necessary due to the social order of the time, but could lead to motley forces with variable training, equipment and abilities. The more resources the noble had access to, the better his troops would typically be.