The army aggregates the various means put in place by a country for its protection and that of the values it defends. When speaking about an army, we usually include all various armed forces, which are the marine, the army and air forces.

Out of those three armed groups, the army is probably the oldest force. Ever since Antiquity, the army enabled some nations to defend themselves and others to attack. The Spartan Army is possibly one of the oldest professional armies of all times. Young men started training hard and intensively at 7 years old. Logically, they had to serve loyally up to 60 years, that is, up to their retirement. However, only a few of them reached that age.

In addition to national armed forces, there are mercenaries and Private Military Companies (PMC). National armed forces are usually made up of carrier officers or enlisted soldiers. Depending on the needs, especially in case of a natural disaster or of a looming war, an army may call upon reservists. Reservists receive a quasi-similar military training as carrier officers. But reservists are civilians, who work for the state or even for private companies throughout the year. They are called for special missions or in cases of crisis.

In most countries, an army is made up of the land-based armed forces, of the marine (naval forces) and of air forces. Sometimes these three categories are independent in their fields of expertise. But, most often now, they play an integral part in a joint warfare organization directing the operations.